Abstract: If the inrush will occur it can be judged according to the fracture mechanics.But the value of the critical stress intensity factor must be measured firstly.Because of the complexity of the rock structure, the suitable rock sample is very difficult to obtain.In-situ stress measurements by hydraulic fracturing is explained according to the fracture mechanics theory,and the fracture mechanics model is set up.The computation equation of the critical stress intensity factor is deduced.The material of in-situ stress measurements by hydraulic fracturing in Gedian coal mine is analyzed applying the theory.The critical stress intensity factor is calculated applying the equation.