Excavation of the expansive soil slope results in fissure development due to drying-wetting cycles, and affects the stability. Although the materials of cement panel and geotextile are generally used to deal with the drainage problem successfully, their service lives are short and less ecological friendly. Therefore, a capillary barrier with dual soil structure consisting of a relatively fine soil layer above a relatively coarse soil layer, is introduced in this paper considering both ecological and drainage-seepage controlling functions. The aim of this article is to investigate the effects of particle size and distribution of the coarse soil layer on the capillary seepage flow. Six groups of tests are conducted to monitor the soil water infiltration process under the condition of rainfall using the brilliant blue dyeing tracer. The results show that preferential flow and discontinuous horizontal seepage field appear in coarse soil with larger particle size and worse particle distribution, and worse capillary barrier effect. The measured vertical and horizontal drainage volumes indicate that the dual structure presents good drainage control. The parameter of steady drainage efficiency and the total drainage efficiency are used to evaluate the effect of capillary barrier. The steady drainage efficiency between 80% and 100% is small firstly and then increases with smaller grain size and better particle distribution of the coarse soil layer. The variation of total drainage efficiency between 43% and 79% is just opposite compared to the former. In summary, the dual structure of the fourth scheme is suggested to apply the South-to-North Water Transfer Project, according to the optimal seepage control efficiency and the lower cost of coarse material. The research results can be directly applied to the seepage control of expansive soil slopes in other projects.