The complex mechanical properties of soft broken rock mass lead to a series of challenging issues in deep mining engineering. In-depth study of the post peak mechanical behavior of broken rock mass is significant for analyzing the stability of surrounding rock and designing support structures in deep roadway. By considering the effect of confining pressure, a mechanical model of the post peak strain softening is established on the basis of a quantitative geological stremth index (GSI) evaluation system of surrounding rock and the continuous media theory. The reliability of the proposed model is verified by comparing numerical results with an application of a field engineering project. By calculating the relationships between surrounding rock and support structure, it is indicated that calculated results by the ideal elastic-plastic model and by the strain softening model are obviously different. The use of ideal elastic-plastic model may lead to a low safety and even instability of the support structure. Field application results are in good agreement with numerical results. The proposed model can reflect the nonlinear failure behavior of broken rock mass and provide new insights into the stability analysis of surrounding rock and the design of support structure.