The birth intervals,as an important factor of understanding the fertility level and the mobility of reproductive behavior of the floating population,have not been sufficiently discussed in the existing scholarship.This article analyzes the birth intervals of the rural-urban floating population with the megacities dynamic monitoring data of the floating population provided by National Health and Family Planning Commission of the Peoples Republic of China,using Cox regression model.The result shows that floating delays a)the first marriage age,b)the first,the second and the third birth age,and c)the first,the second and the third birth interval.It is also found that women of childbearing age,who have given birth in their original area,tend to delay the latter birth interval after floating into the destination.The reasons are the tension,fatigue,and the unsteady state imposed by the unfamiliar place,when they start a new life in the floating destination,that is,both the physiological and psychological burden interferes with a womans fertility behavior.