The Composite prestressed soil nailing retaining technique is widely used for deep excavation in China, but at present the safety factor method is adopted to evaluate the stability of this retaining pattern. This valued evaluation method doesn't take account of the space distributing properties of soil's parameters. In order to offer an exact excavation index of the stability of this retaining pattern, this article defines the effect of prestressed bolt to soil nailing retaining wall, adopts reliability analysis method to carry out the reliability analyses of the stability of this retaining pattern, and gives out the failure probability of soil nailing retaining wall. By analyses of engineering cases, the practicability of the analysis method is verified.