Tunnel face stability is one of important issues affecting the safety of underground engineering construction. The limit analysis based on the upper and lower bound theorem is one of the commonly used methodologies for analyzing the stability of the tunnel face. Among such methods, the adaptive mixed constant-smoothed strain limit analysis (MCSE-LA) method has been proved to have certain advantages over traditional finite element limit analysis in terms of computational accuracy, efficiency, and convergence. Based on it, the feasibility of the adaptive MCSE-LA in the stability analysis of the tunnel face is verified first, and it is found that the stability index (stability number) of the tunnel face obtained by this method is the same as that of the traditional finite element limit analysis. At the same time, the failure modes of tunnel face can also be obtained by adaptive mesh refinement based on the maximum shear strain rate. In addition, this method is also used to analyze the influence of the length of unsupported section of the tunnel excavation on the stability of the face, and the change law of the stability number with the increase of the length is analyzed and summarized, which has certain significance for guiding the construction of underground engineering.