Abstract: Using elastic thin plate theory, corresponding mechanical models for main roof at mining face during different stages are established, and the mechanical formulas of main roof during the first weighting and periodic weighting are got. Combining with the boundary conditions of rock unstable, the first weighting and periodic weighting pace can be calculated directly. According to this calculation method, the roof weighting distance in Zhangji Coal Mine 1215(3) working face have been verified, and the relationship between the main roof weighting pace and several main factors has been analyzed.