To reveal the reinforcement mechanism of pipe shed support on the tunnel face of soft rock tunnel and evaluate the stability of the tunnel face under pipe shed support, the numerical simulation method is used to compare and analyze the failure mode of tunnel face with or without pipe shed support. Then, based on the numerical simulation results, a new parabolic arch-logarithmic spiral tunnel face failure model considering pipe shed support and soil arch effect was proposed, and the calculation method of tunnel face safety factor under pipe shed support was derived by upper bound limit analysis method and strength reduction method. Finally, the influence of surrounding rock parameters and pipe shed support parameters on the stability of tunnel face was discussed. The research results indicate that when the instability failure of the tunnel face without pipe shed support occurs, the surrounding rock above the tunnel face forms a large collapse arch and a small collapse arch. Because of the pipe shed support, the instability failure of the tunnel face dose not develop to the vault. Meanwhile the pipe shed support prevents the formation of local collapse arch above the unsupported section, and makes the displacement direction of the surrounding rock above the unsupported section change from vertical downward to extru-sion to the tunnel face. The safety factor of tunnel face stability increases with the increase of internal friction angle, cohesion of surrounding rock and the size of pipe shed, and increases with the increase of single excavation length and pipe shed spacing. Too large size of the pipe shed is not economical for enhancing the stability of the tunnel face.