Rock is a type of natural material containing microcracks. The evolution of mining-induced fracture field is one of the main problems of gas comprehensive control and rock mass seepage. The distribution of fissures and porosity in overlying strata under the mining influence were studied in this paper. The stratum subsidence was represented quantitatively using elastic thin plate theory and key strata theory. Based on the stratum subsidence, a program calculating porosity in overlying strata was developed based on VB. The results demonstrate that the porosity increase with face advances. The porosity becomes small with the decrease of strata depth, and the bigger separation is located in the strata below the key strata. The amount of the fissures in the middle part of overlying strata is more than that in two ends, and the porosity curves present symmetric arch shape in the initial stage. With face advances, the curves of porosity in the strata located the upper position change symmetric arch shape into saddle shape.