Abstract: The rock mass rating (RMR14) method can evaluate the mechanical properties and structural conditions of rock mass comprehensively and consider the engineering factors such as the initial ground stress fields and excavation ways. RMR14 is quite suitable for evaluating the surrounding rock of mountaneous tunnels. But the ladder-like rating in RMR14 will cause the fuzzy uncertainty at the interval boundaries. In order to evaluate the quality of rock mass more accurately, the fuzzy membership functions are introduced to solve the problem of fuzzy uncertainty based on the Mamdani fuzzy inference method. A new RMR14 method based on the fuzzy inference is proposed by using the “if - then” reasoning rules to realize multiple input parameters and inference rules of the fuzzy inference. The new method is used to assess the qualities of the surrounding rock mass of Piaoli tunnel of Dushan-Pingtang highway. The results show that the new RMR14 based on the fuzzy inference can draw the existing engineering experience and appropriately solve the boundary problem of fuzzy uncertainty. Compared with the original RMR14, the new RMR14 based on the fuzzy inference can assess the quality of the surrounding rock more accurately and steadily.