Abstract: Based on the methodology of study on water, soil environmental capacity and mass conservation theory in a system, the concept and model on sediment environmental capacity for ¹³⁷Cs in Daya Bay are developed. The static capacity for ¹³⁷Cs in upper layer of sediment near shore at shallow area is calculated, and the annual dynamic capacity and total dynamic capacity are given through determination of typical biomass in the sediment. The results show that the estimated environmental capacity for ¹³⁷Cs in sediments is approximately equal to current input of ¹³⁷Cs into the sediments, and to control the input of ¹³⁷Cs in the sediments within the environmental capacity is the guarantee for sustaining of current situation of Daya Bay eco-system and avoiding of obvious degradation of the eco-system.