Abstract: Mining effect plays key role to control deformation of the surface，and whether value of subsidence coefficient is precise or not can directly determinate the accuracy of the prediction results of surface movement deformation．In order to discuss the relationship between subsidence coefficient and mining range under thick alluvium，based on the observation data’s of surface movement observation stations under thick alluvium，the influence degree of thickness of alluvium to supercritical mining degree was studied，the unreasonable reason was proclaimed that tradition method estimates mining degree，then stress distribution of overlying strata and movement mechanism caused by mining were analyzed under thick alluvium．Then the estimation criterion that the bedrock and alluvium is regarded as the different composite medium was provided for mining degree，by calculated and analyzed，the critical mining width was proposed for supercritical mining，and subsidence coefficient was obtained under thick alluvium. Compared with actual measurement，subsidence coefficient was in accord with actual situation comparatively，which provided science prediction for“under railways，under water bodies and under buildings”under thick alluvium in mine-area．

Key words:  , thick alluvium；subsidence coefficient；surface movement；degree of mining effect；critical mining width