Abstract: Base on general three dimension nonlinear numerical simulation software,mechanism of composite soil-nail retaining structures in foundation-pit engineering are analyzed in this paper.Research contents include whole construction process simulation of excavation by steps,soil nailing installation and panel manufacture etc.By the comparison of mechanical characteristics among the pure soil-nail retaining support,pre-stressed anchor composite soil-nail support and pre-reinforced micro pile composite soil-nail support in the same engineering condition,tension distribution range of soil layer,bottom heaving in the pit and axial force distribution along nail etc.are analyzed.The action mechanism and working performance of soil-nail retaining support are also systematically studied and theoretical basis can be provided in optimal design,popularization and application.