Abstract: For the problem of reliability analysis of stability of composite soil-nailing supporting structures with prestressed anchors, based on the limit equilibrium theory, the safety coefficient formulas and reliability analysis functions for foundation pits are established. Then taking the mechanical parameters of soils as random variables, the stability reliability index and failure probability of foundation pits are calculated using the Monte Carlo importance sampling method. A project example is used for analysis. The results show that the formula for the stability calculation accords with the demands of engineering and the reliability analysis, so its use for calculating the stability of composite soil-nailing structures is effective. In the reliability analysis of the foundation pit, the reliability index and the failure probability are greatly influenced by the cohesive force, internal friction angle of soils and friction resistance of coefficient of variation. The former decreases with the increasing coefficient of variation, while the latter increases with the increasing coefficient of variation. For the selection of distribution of mechanical parameters of soils, the cohesive force of parameters takes logarithmic distribution, and the angle of internal friction and the friction resistance adopt normal distribution. The reliability of the calculation and analysis is quite reasonable.