By simulating the unstable seepage field of the foundation pit during precipitation, the variation law of water pressure acting on the cement-soil retaining wall during precipitation is analyzed, and the influence of unstable seepage on the stability of such retaining wall is discussed. The analysis results show that the water pressure around the wall decreases with time, while the sub-coefficients of overturning resistance and horizontal sliding resistance increase continuously, and are significantly larger than the calculation results without considering seepage, two-dimensional stable seepage and one-dimensional simplified seepage. Therefore, the influence of unstable seepage should be considered in the design of foundation pit support.