Abstract: Understanding the shear dilation of rock joints under constant normal stiffness boundary conditions is crucial for the stability of deep underground engineering rock mass.In this paper,the UDEC software was used to construct a discrete element calculation model for rock joints under constant normal stiffness boundary conditions based on a continuously-yielding micro-segment joint model.The dilation effect of rock joints was discussed.The results show that the dilation displacement of rock joints dilation shows a trend of negative exponential growth under constant normal stiffness boundary conditions.The maximum dilation displacement of rock joints is positively correlated with the maximum undulation height of rock joints and negatively correlated with the initial normal stress and normal stiffness.The dilation convergence rate of rock joints increases with the increase of initial normal stress and normal stiffness.Through the analysis of shear dilation numerical curves of rock joints,a nonlinear dilation model under constant normal stiffness boundary condition considering the maximum height fluctuation of the joint surface,asperity dip angle and initial normal stress was proposed.The comparison of the dilation theory curves and the numerical test curves,shows that the model has good universality.