Abstract: In order to study the dynamic distribution law of Overlying strata fissures during the mining process,the UDEC software is used to study the stress,displacement and fracture development of the overburden layer on the working face during the mining process.The results show that with the mining of coal seam,the subsidence of overburden gradually increases,the vertical displacement of the coal seam roof changes the most,and the range is also the largest.The farther the overlying rock is from the coal seam,the smaller the subsidence.A large number of cracks are formed in front of the openoff cut and behind the work surface,while in the middle of the goaf,the crack distribution is less.According to the fracture condition,the three zones are divided into three heights,among which the collapse zone is 17 m high and the fracture zone is 64 m high,which is consistent with the derivation of the empirical formula.The porosity distribution was calculated,and the square porosity in front of the cutting face reached 0.25,and the rear porosity in the working face reached 0.35 Combined with the formula,the threedimensional orosity variation diagram of goaf can be obtained.

Key words: Overburden rock, Crack, Displacement, Porosity