Abstract: Accumulation effect of dynamic stress is put forward for the first time according to model simulating test.It will take a long time for the dynamic stress in plastic soil caused by repeated dynamic loads to release absolutely,and part of the dynamic stress will gradually add to the dynamic stress caused by next dynamic loads,which is the "accumulation effect" of dynamic stress.The accumulative dynamic stress lies on soil’s character,dynamic stress intensity caused by the loads,frequency and times of the loads. The dynamic stress in subgrade soil under repeated dynamic loads composed of stress caused by single load and accumulated stress caused by repeated loads.