Abstract: Taking the tunnel construction near the Gaojiawan fault fracture zone as the engineering background, the shear instability mechanism of the outburst-prevention rock mass at the working face in the deep and large water-rich fault fracture zone tunnel was studied by theoretical analysis and numerical simulations, and the safety thickness of the outburst-prevention was analyzed. The results showed that the essence of water and mud inrush in tunnels near the water rich fault fracture zone was that construction disturbance leaded to the change of the surrounding rock stress fields and seepage fields in the fault fracture zone. Under the water and force coupling effect, the insufficient outburst-prevention thickness of the rock mass at the at the working face in tunnels caused shear instability; the changes of instability evaluation indexes such as fault zone water pressure, water inrush volumes at working faces of tunnels, horizontal displacement and plastic zone areas all reflected obvious characteristics of "stability—development—instability" in the process of tunneling through the fault fracture zone, which were also basically consistent with the development process of water and mud inrush in actual construction. Integrating various instability evaluation indexes, it was determined that the critical safety thickness of the tunnel for outburst prevention was 0.95 times the effective diameter of the tunnel. Compared with the width of the fault fracture zone, the burial depth had a more significant impact on the outburst-prevention of tunnels in the deep and large water-rich fault fracture zone. Simultaneously, with the increase of the tunnel burial depth, the influence of the width of the fault fracture zone on the safety of outburst prevention gradually increased.