Abstract <p>The integrity and mechanical properties of rock mass are significantly impacted by the size of discontinuities. The trace length is an vital index to quantify the size and continuity of discontinuities, and it also stands as an essential parameter for discontinuity network simulation and geomechanical analysis. The existing rock mass discontinuity mean trace length estimation methods mainly concentrated on the planar outcrops, while no attention has been paid to methods applicable to non-planar outcrops that are more universally existing in nature. In this paper, the rock mass discontinuity mean trace length estimation method based on planar windows was theoretically improved to be applicable to two typical windows on non-planar outcrops, curved non-planar windows and folded non-planar windows. A projection technique was imported in the proposed mean trace length estimation method and the spatial shape of the windows was considered by defining a new apparent dip angle on the considered non-planar sampling windows. The proposed method was applied to estimate the rock mass discontinuity mean trace length. The proposed method was applied to calculate the rock mass discontinuity mean trace length by using discontinuities appearing on an outcrop in Mindu area of China by placing a series of non-planar sampling windows in different locations. Traditional estimation method based on planar sampling windows was also used for comparison. Results indicate that the predictions based on the method proposed in this paper are more accurate than that based on traditional planar estimation method. This work contributes theoretical research level of geometric parameter estimation of rock mass discontinuities appearing on irregular natural outcrops.</p>