<p>Accurately estimating the mean trace length (<i>mtl</i>) of discontinuities is crucial for three-dimensional (3D) network simulations of random discontinuities. Currently, the <i>mtl</i> is mainly estimated by laying out scan lines or sampling windows, based on the intersection of discontinuities with these scan lines or sampling windows. However, these methods project discontinuity traces interpreted by 3D point clouds or models onto a two-dimensional (2D) sampling window, which leads to the loss of the trace length. This study aims to address this limitation and proposes a 3D mean trace length estimation model (3D-MTLEM) by extending the 2D rectangular sampling window to a 3D cuboid sampling block, enabling the full utilization of the 3D discontinuity information. Firstly, the proposed 3D-MTLEM is applied to three simulated trace datasets. The results indicate that compared to traditional 2D window estimation methods, the 3D-MTLEM provides more accurate and stable estimates of <i>mtl</i>. Furthermore, the 3D-MTLEM conducts an analysis of the influence of four key factors: trace length (<i>tl</i>), trace direction (<i>td</i>), trace length distribution function (<i>tldf</i>), and cuboid sampling block size (<i>csbs</i>). Subsequently, the 3D-MTLEM is applied to two case studies of an artificial quarry slope, further confirming its applicability in practical engineering.</p>