Abstract: To solve the problems in coal mining under buildings，railroads and water bodies and improve recovery of coal resources，the strip-filling parameters in a coal mine are investigated.According to the key strata theory，the limit span of roof for the "under-three" coal seam is calculated.It is known that the fracture of basic roof can not occur in this coal mine when the width of unfilled area is less than 41.8 m.Based on this，three strip-filling programs are analyzed by numerical simulation software FLAC^3D to research the plastic zone and stress distribution of strip-filling under different width of backfill and the fixed filling space.The results show that the filling rate should be controlled at 70% or less.Through the comparison and analysis of different simulation programs，the ratio of plastic zone in strip-filling is about 30%，and the elastic core of backfill is of long-term stability，when the filling rate is 55.5%.Practices show that the destruction of surface buildings is within the range of gradeⅠwhen "under-three' mining happens under these strip-filling parameters.It is significant of using waste rock to achieve the green mining.

Key words: "Under-three", coal mining, Filling strip, Numerical simulation, Parameter optimization, Stability analysis