This paper is based on the method of random angle generating curve sliding surface. It analyzes the stability of slopes comprising different soils under the action of seepage. It changes the value of the selected parameters to search for critical sliding surface. Mechanical analysis is conduced to derive the formula of buoyancy and penetration of slices in simplified Janbu method. The results show that fristly, both the minimum factor of safety and the most critical sliding surface obtained by this method are very close to the results of the existing methods, which shows the correctness of this method. Secondly, the minimum factor of safety calculated by using non-circular sliding surface method is significantly smaller than that by using circular sliding surface method, which recommends that the non-circular sliding surface method be adopted under action of seepage. Thirdly with increase of groundwater level, factor of safety of slopes was sharply declined. So effect of seepage is an important factor for occurring of slope instability. Setting of drainage facilities on slopes should be paid special attention in engineering practice.