Abstract: In order to control the size effect of bolt-mesh support for roadways in extremely thick coal seam, simulation studied the rock surface displacement of 16 different models which bolt length was 2.0,2.2,2.4 and 2.6 m,and cable length was 5.0,6.5,8.0 and 10.0 m with FLAC^3D,and fitted the calculation results and drew its graphics with MATLAB, the results showed that the distance between two rocks or roof and floor and bolt, cable length showed a cubic function, and their graphics was a "scallop" shape; the effect of bolt-mesh support is determined by bolt, cable joint action, it should be paid attention to the synergy between bolt and cable, and made the performance of both supporting full play, bolt and cable length should be 2 400 mm and 8 000 mm in 5203 roadway supporting; industrial tests show that distance between two rocks or roof and floor were 83 mm and 132 mm during drifting, and it was consistent with the findings.