Besides the loading applied by the upper building, the piles are often subjected to indirect loading such as side loads. These kinds of loads would cause the pile bear additional force (negative skin friction), which may be large enough to reduce the bearing capacity and cause structural failure of a pile. By establishing the numerical model of pile soil interaction, the bearing capability of pile under side load was analyzed. The joint effect of different magnitude and the distance of the side load on the skin friction were focused. The joint effect of length of the pile and the distance of side load on the skin friction was discussed. The expression of the two combination relationship was obtained. Meanwhile, by using the least square method, the expression of total skin friction on different side load combination was achieved. And based on the statistical analysis of the dispersion coefficient, the function expression was proved to be of high precision. The results show that when k, which indicates the relative relation of side load distance s and the side load magnitude Q, increases to 32, the value of negative skin friction goes to zero. When c, indicating the relative relation between pile length L and side load distance s, increases to 0.5, the value of negative skin friction goes to zero. Beyond limit value of these two relative relationships, the bearing capability of the pile will not be affected by side loads.