The traditional rainfall infiltration boundary, decomposing the rainfall intensity according to the slope direction, does not conform to the actual non-orthogonal rainfall infiltration. Firstly, for the purpose of studying the rules of non-orthogonal rainfall infiltration on the unsaturated silty clay slopes, the defects of the slope infiltration models and boundary conditions now available were revealed and analyzed by the review of infiltration theories. Secondly, the infiltration tests of unsaturated silt clay slopes under different rainfall intensities, slope angles, and void ratios were carried out by designing an artificial rainfall device. The test results indicate that the rainfall infiltration on surfaces of unsaturated silt clay slopes is non-orthogonal infiltration rather than simply decomposed orthogonal infiltration; for a given soil slope with a definite angle of and a void ratio, there exists an optimum rainfall intensity at which the maximum infiltration can be achieved; the smaller the void ratio is, the faster the infiltration rate changes at the beginning stage of a rainfall and quicker to come to its stable state. There is an optimum slope angle at wich the amount of infiltration is the smallest, which indicate that the rate and total amount of infiltration are not vary with the slope angle monotonically.