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To better understand waterlogging effect on leaf senescence in winter rapseed（Brassica napus L.） during flowering stage, experiments were designed to explore foliar K application influences on adverse effects of waterlogging stress. Winter rapeseed was sprayed with K after waterlogging at initial flowering stage. Results indicated that waterlogging significantly decreased leaf net photosynthetic rate（Pn）, stomatal conductance（Gs）, intercellular CO2 concentration（Ci） and transpiration rate（Tr）. It also declined maximum quantum yield of PS II（Fv/Fm）, quantum yield of electron transport（ΦPS Ⅱ） and photochemical quenching（qP）, but increased leaf non-photochemical quenching（NPQ） and minimal fluorescence（Fo）. Interestingly, exogenous application of K significantly alleviated waterlogging-induced photosynthesis inhibition. Foliar K application increased RuBisCO activation, chlorophyll and soluble protein contents, while significantly decreased MDA content under waterlogging stress. Moreover, K supplementation improved accumulation of K+, Ca2+, Mg2+, N, Zn2+, Mn2+, Fe2+ in leaves. In general, foliar K application is effective in alleviating deleterious effects of waterlogging stress and delays leaf senescence of winter rapeseed.