Abstract:  In Qian Jiahuan's load-unload elastic model of dynamic compaction,according to the disadvantages that the impact stress is large at the loading stage and the stress style is complicated at the unloading stage comparatively,the complicated piecewise function is improved into standard sine function. And,on the basis of the relationship between stress boundary time-history and speed boundary time-history in dynamic analysis,the stress time-history of dynamic boundary condition is transformed into the speed time-history and the sine load function is introduced. A new simplified method under the sine load is derived,and it can calculate vertical displacement well. Engineering practices show that the method is simple and easy to use,and the calculated results are very close to the measured displacements