Abstract: The filling effect of lower coal seam directly affects the surface movement and deformation in the multi-coal seam mining.The higher ratio of backfilling is,the more obviously the effect of surface subsidence decreases,but the higher the mining cost becomes.In view of this contradiction，and based on the FLAC3D numerical simulation software,the change law of the surface movement and deformation were studied systematically for different size of ratio of backfilling.The experimental results showed that：①With the increase of ratio of backfilling,The value of surface movement and deformation is decreasing in negative exponent function,and no longer changes until it reaches a certain value.②When the ratio of backfilling is 80%,the maximum surface subsidence value is 750 mm，the maximum slope deformation value is 1.5 mm/m，the maximum horizontal displacement deformation value is 1.4 mm/m，and all of them conform to the requirements of the regulations.Then,the safety of ground buildings(structures) can be ensured,and also the optimal economic benefits is achieved.The research results have important theoretical and practical significance for guiding backfilling mining of lower coal seam.

Key words: Strip mining, Ratio of backfilling design, FLAC3D, Surface movement and deformation