The developed joint and poor quality of rock mass are the direct factors affecting the stability of stope in Bainiuchang mine.In order to obtain the actual mechanical parameters of rock mass and accurately evaluate the safe span of stope,the extended Hoek-Brown criterion was partially modified in this paper.Then,the traditional determination method of GSI was improved by introducing parameter Js(joint spacing in rock mass),so to provide a more quantitative description of the structure of jointed rock mass.The various mechanical parameters of rock mass were treated by adopting classical formula of Hoek Brown criterion.Finally,the mathematical relation of geological strength index GSI to RMR and Q was derived.The safe spans of stope in six kinds of rock mass were evaluated by using the empirical formula of engineering span proposed by Barton.It was concluded that the safe span of underground stope without support is not more than 10 m.The results show that the proposed method provides a simple but convenient way to evaluate the mechanical parameters of jointed rock mass,and the estimated safe span of stope is also accord with practice of production,so the proposed method can be applied to similar mines.