Abstract: This paper quantitatively evaluates tunnel stability using a safety factor defined in light of rock deformation and discusses the degree of stability by increasing the actual rock displacement caused by tunnel excavation to the ultimate displacement at the critical state. Based on the theoretical elasto-plastic analysis of an unlined circular tunnel, it derives a principle parameter reduction method in which the increase of actual displacement is achieved by considering the elastic modulus degradation of the surrounding rock. The safetyfactor is calculated by using the ultimate displacement as the criterion of tunnel instability. With the finite-element parameter reduction method, the stability of unlined circular tunnels with different depths and diameters in various grades of rock is discussed. The results show that the parameter reduction method is feasible and applicable to rock masses of grades I to IV, whereas there are some deviations for the rock mass of grade V.