Soil-water characteristic curve and hydraulic conductivity function are determined based on VG model, and then by way of constructing finite element numerically analysis model, both seepage-stress coupling analysis and uncoupling analysis which only considering seepage are conducted by using finite element software SIGAM/W and SEEP/W respectively, factors of safety is calculated by software SLOPE/W using seepage field and stress field from coupling analysis and uncoupling analysis and analyze stress and strain to research the principle of coupling effect on slope stability. The results show that the rate of rain infiltration advance is faster due to seepage-stress coupling effect which results in faster loss of suction compared with uncoupling analysis. Rainfall has negative influence on slope stability,the factors of safety from coupling analysis is lower than that from uncoupling analysis, thus the influence of coupling effect on slope stability cannot be neglected. Stress concentration appears in surface layer and stress concentration belt appears at the depth of wetting front on the condition of rainfall infiltration, which means that slope is prone to shallow failure. Slope toe is the first region to failure because horizontal displacement in slope toe is higher than that in slope top.