In the design of a pile-anchor supporting system of a foundation ditch, adequately designing the embed depth of the supporting pile is very important. Generally, a deterministic model of the simple arc sliding strip method is used to calculate the overall stability of the foundation ditch, which is a constant value. As the uncertainties of loads and soil properties are not considered, the actual overall stability may exceed the calculated result. In this paper, first, based on a typical pile-anchor supporting system of a foundation ditch, the deterministic model of the simple arc sliding strip method is introduced to calculate the overall stability. Based on the deterministic model, the limit state equation and reliability model of overall stability for the pile-anchor supporting system of a foundation ditch is developed by considering the loads and the soil parameters as independent normal distribution variables. In addition, the reliability index of overall stability is calculated using the first order reliability method. The target reliability index is used to determine the embed depth of the supporting pile. Finally, through a specific pile-anchor supporting system of a foundation ditch, the safety factor and reliability index of the overall stability supporting pile were calculated using the safety factor method and reliability method, respectively, to determine the embed depth of the supporting pile. Also, the influence of coefficient of variations of calculating the parameters for the reliability index of the overall stability is analyzed. Through this study, it can be known that the embed depth of the supporting pile designed using the deterministic method may damage the foundation ditch. The embed depth of the supporting pile designed using reliability analysis is more safe, reasonable, and economical since the uncertainties of the loads and soil properties are considered. Besides, the variation of parameters c, φ, and q_s has obvious influence on the reliability index of the overall stability and pile length. Therefore, when designing the embed depth of the supporting pile in a foundation ditch, reliability analysis should be used together with the simple arc sliding strip deterministic method.