Based on Yangshan Phase IV project,we analyze and compare the test results of the backfill layer (sand soil),and study the physical and mechanical properties of backfill layers under preloading.According to the tests of the density,strength and deformation characteristics of the backfill layer of different preloads and periods,we reveal the variation rules of them with the load and time of the backfill layers to serve as technical reference for estimating the scope,depth and determining the foundation reinforcement technique of the backfill layer.The analysis results indicate that with the increase of the load and loading time,the engineering geological properties of the backfill layer are improved obviously,and the improvement effect is more obvious than that by the self-weight.