Abstract: Core mechanical experiment and block in?situ stress modeling along with theoretical analysis, numerical simulation and case study have been conducted to investigate the shear fracture mechanism of natural fractures during fracturing in ultra?deep fractured reservoir. It is suggested that natural fractures become shear failure when pore pressure reaches the natural fracture opening threshold, and the natural fractures with weak surface cementation and the dip angle of nearly 60° are more likely to happen for such a shear failure. Pumping pressure and pore pressure assume a relationship of linear increase, but all natural fractures are difficult to achieve fully opening. Hydraulic fractures tend to opening along natural fracture development segment in the current working conditions. The failure of rock is hard to make by fracturing and some natural fractures in a favorable orientation can not be opened. By optimizing pump rate, sand volume and pumping procedure, the stimulated reservoir volume or fracturing volume can be improved, and better results have been obtained in field application