Abstract: The stage empty field subsequent filling mining method was adopted for mining，the ore body below -530 m of a certain iron mine.Through the methods of indoor tests，engineering analogy and theo⁃ retical calculation，the strength of the required filling body in the deep part was analyzed to provide a reason⁃ able filling slurry ratio for the mine. The results show that when graded tailings are used as the filling aggre⁃ gate，the slurries with mass concentrations of 64%，67%，and 70% have good fluidity，and the slump of the slurries is all greater than 210 mm，which meets the transportation conditions in the pipeline.The uniaxial compressive strength of the filling body maintained for 28 d under the conditions of ash-sand ratios of 1∶6，1 ∶8，1∶10，and 1∶12 respectively is relatively high.Moreover，with the increase of mass concentration and ash�sand ratio，the strength of the filling body increases accordingly，with the lowest being 1.596 MPa and the highest reaching 8.039 MPa，which can provide a good filling effect for the filling of deep empty areas. Combined with the deep mining conditions and filling techniques，the strength of the deep filling body in the mine needs to meet the requirements of R28≥3.0 MPa in the first step and R28≥1.5 MPa in the sec⁃ ond step. Filling slurry with a mass concentration of 67% can be adopted，among which the ratio of ash to sand in the first step filling is 1∶10 and that in the second step filling is 1∶12.

Key words: classified tailings, cemented filling bodying, filling body strength, stage empty field sub? sequent filling mining method, deep mining