Abstract: The Wilcoxon rank sum nonparametric statistical test is introduced to establish an evaluation model for identifying structural domain boundaries in jointed rock masses. A new method for identifying structural domain boundaries considering discontinuity orientation and trace length is proposed on the basis of random mathematics. The Wilcoxon rank sum test is used to calculate the correlation between samples collected from four adjacent adits at different elevations of Maji Hydropower Station at the left bank of Nujiang River in Yunnan Province of China so as to rationally divide statistical homogeneous areas of discontinuous rock masses. First, the Schmidt plots are divided into 100 windows with approximately equal areas. Second, the poles occurring in each corresponding window are arranged in ascending order with respect to their trace lengths. Finally,the Wilcoxon rank sum test is used to identify the homogeneity of structural populations by analyzing the discontinuity sequence. The results indicate that, PD201 and PD251, PD251 and PD253 are situated in different structural domains, while PD241 and PD253, PD201 and PD241 in different areas have different homogeneous area evaluation results, indicating that the statistical homogeneous area has size effect. The Miller’s method only considers discontinuity orientation, while the new method is more reasonable becomes it considers both the orientation and the trace length. The other advantage of the new method is that it can be employed to evaluate statistically homogeneous regions with small sample sizes.