Abstract: The traditional rainfall infiltration boundary, which is determined by decomposing the rainfall intensity simply according to the direction of a slope surface, does not conform to the actual non-orthogonal rainfall infiltration. In order to study the rules of non-orthogonal rainfall infiltration on the surfaces of unsaturated sand slopes, a series of infiltration tests on unsaturated sand slopes under different rainfall intensities, slope angles and void ratios are carried out using a new and self-designed artificial rainfall device. In addition, numerical simulations under the same conditions corresponding to the laboratory tests are employed to analyze the rainfall infiltration. The variations of infiltration rates, seepage rates and water storage increments of sandy soil with respect to infiltration time are obtained, and the influences of the rainfall intensities, slope angles and void ratios on the test results are analyzed. The test results show that the overland flow does not emerge in all the tests. They are different from the orthogonal infiltration theory. Finally, the comparison between numerical simulations and test results unveils the differences in infiltration rates, seepage rates and water storage increments of sandy soil, and demonstrates that the moisture content of the soil slope, water infiltration capacity and the transformation mechanism of boundary conditions, which are calculated with the orthogonal infiltration theory, do not conform to the actual cases of non-orthogonal rainfall infiltration.