Abstract: Based on soil nail axial force and slope displacement testing, a 3-D soil nailing support numerical model is constructed and the slope displacement, soil nail axial force, plastic displacement are deeply discussed in this paper.Most meaningful conclusions are drawn: soil nail and soil mass form a whole supporting system through their interaction; foundation pit in stability is not due to soil mass failure or soil nail failure, but is due to the instability of whole supporting system; the supporting system is formed by soil nail bearing partial force and stress transferring and diffusing. These are the essence of the soil nail support.