[Purposes] This paper aims to deeply investigate the method for determining the roughness coefficient of silty mudstone joint surfaces (JRC) under different shear directions and stress states. [Methods] Materials similar to silty mudstone and 3D printing technology were used to fabricate joint surfaces. Direct shear tests were conducted on the joint surfaces under different stress states. Furthermore, based on the morphological parameters of joint surfaces proposed by Grasselli, the shear direction was considered to investigate the shear strength of silty mudstone joint surfaces. [Findings] The failure types of silty mudstone joint surfaces are different due to the fluctuation degree of joint surfaces, shear direction, and normal stress, and the JRC value obtained by inversion of test results is larger than the empirical JRC value proposed by Barton et al. In addition, a method of calculating JRC value of silty mudstone joint surfaces considering direction and stress state was proposed. The error between the predicted value and the calculated value is less than 10%. [Conclusions] The roughness coefficient of joint surfaces has a significant relationship with the undulation pattern of shear direction. With the increase in normal stress, the JRC value increases, but the rate of increase decreases. The JRC value calculation method considering the shear direction and normal stress of the joint surfaces can effectively predict the shear strength of silty mudstone joint surfaces under different shear direction and normal stress conditions.