The fundamental assumption of ~(210)Pb_(se)diment dating is the stable flux of ~(210)Pb_(se), which was derived from atmosphere and then transferred into sediments via lake water. When the sedimentation rate is relatively constant, the ~(210)Pb_(se) activity in sediments will be exponentially reduced with sedimentation age. ~(210)Pb_(se) in lake water is incorporated into sediments mainly via organic particulates. If the sedimentation flux of organic matter in lake water is suddenly in- creased, ~(210)Pb_(se) will be significantly deposited and then transferred into sediments. On the one hand such sudden purification effect is obviously unfit for the fundamental assumption of ~(210)Pb_(se) dating; on the other hand, the sudden enhancement of ~(210)Pb_(se) flux would be indicative of the conspicuous variation of primary productivity of lake water. This problem will be discussed in accordance with the variation trend of 210Pbex in the vertical profile of recent sediments of Lake Chenghai, Yunnan Province. The sediment core was collected from the deep-water area of Lake Chenghai in June 1997. The vertical profile of 137 Cs activity is characterized by a tree-peak pat- tern. This profile gave reliable ages, and also showed the stability of sediment accumulation in the recent ten years. The vertical profile of~(210)Pb_(se) activity displays a specific distribution of peaks, and is similar to the vertical profile of Corg. This phenomenon seems to be related to the mecha- nism of constraining the transfer of ~(210)Pb_(se) into lake sediments. The average atomic ratios of Horg/Corg and Corg/Norg in Lake Chenghai sediments are 5.51 and 7.04, respectively, indicating that the organic matter was predominantly derived from the remains of endogenic algae. In terms of the three-stage evolutionary characteristics of organic matter in sediments, i.e., “deposition-de- composition-accumulation”, the sedimentation fluxes (F(Corg)) of organic carbon (Corg) since 1970 were calculated by modeling. The sedimentation fluxes of ~(210)Pb_(se) (F(~(210)Pb_(se))) in different years display good synchronous relations with the sedimentation flux of organic carbon (F(Corg)), es- pecially in the years of 1972─1974 and 1986─1989. The variation of F(Corg) led to the variation of F(~(210)Pb_(se)); the variation of F(210Pbex) reflects, to some extent, the historical variation of lake pro- ductivity. The fundamental assumption of ~ (210) Pb se (se) diment dating is the stable flux of ~ 210 Pb se (se), which was derived from atmosphere and then transferred into sediments via lake water. When the sedimentation rate is relatively constant, The ~ (210) Pb se activity in sediments will be exponentially reduced with sedimentation age. ~ (210) Pb se (se) in lake water is incorporated into sediments mainly via organic particulates. If the sedimentation flux of organic matter in lake water (210) Pb se (se) will be significantly deposited and then transferred into sediments. On the one hand hand suddenly sudden effect is obvious unfit for the fundamental assumption of ~ (210) Pb se dating; on the other hand, the sudden enhancement of ~ (210) Pb_ (se) flux would be indicative of the conspicuous variation of primary productivity of lake water. This problem will be discussed in accordance with the variation trend of 210Pbex in the vertical profile of recent sedime nts of Lake Chenghai, Yunnan Province. The sediment core was collected from the deep-water area of ​​Lake Chenghai in June 1997. The vertical profile of 137 Cs activity is characterized by a tree-peak pat- tern. This profile gave reliable ages, and also showed the stability of sediment accumulation in the recent ten years. The vertical profile of ~ (210) Pb_ (se) activity displays a specific distribution of peaks, and is similar to the vertical profile of Corg. This phenomenon seems to be related to the mecha- nism of constraining the transfer of ~ (210) Pb_ (se) into lake sediments. The average atomic ratios of Horg / Corg and Corg / Norg in Lake Chenghai sediments are 5.51 and 7.04, respectively, indicating that the organic matter was predominantly derived from the remains of endogenic algae. In terms of the three-stage evolutionary characteristics of organic matter in sediments, ie, “deposition-de- composition-accumulation”, the sedimentation fluxes (F (Corg)) of organic carbon (Corg) since 1970 were calculated by modeling. The sedimentation fluxes of ~ (210) Pb se (F (~ (210) Pb se)) in different years display good synchronous relations with the sedimentation flux of organic carbon (Corg)), es- pecially in the years of 1972- 1974 and 1986-89. The variation of F (Corg) led to the variation of F (~ (210) Pb se (se)); the variation of F ) reflects, to some extent, the historical variation of lake pro- ductivity.