| Soil nailing supporting is a new technology developed from abroad in recent years, to be used in excavation of soil, slope stability and the new retaining structure based on soil-bearing. The technique had been wide used in our country in nineties last century after it was introduced. However, the mechanism and performance of the new technique was not knew yet at present, so the theory is behind the application.Based on a lot of documents and data and the past research, the strength mechanism of soil nailing was analysed in this paper , including drawing- resistance function mechanism during pulling-resistance test of soil nailing, internal force and displacement of soil nails under normal condition of work, transverse anti-shearing effect of soil nailing, friction reinforcement principle of earth, soil arching effect and apparent cohesion theory. Especially for pulling-resistance test of soil nailing, according to the different directions of frictional force between the soil and soil nails, the test was divided into two period of stages, different mechanics models were utilized to systemically generalize drawing-resistance function mechanism, at the same time the internal force analysis and displacement was derived under normal working state in soil nailing. Soil arching effect was issured in theory and the expression formula of it had been put out and the influencing factors were analyzed. According to the energy method, the formula of the theory based on circle failure mode and per excavated depth is derived. To the stable analyses of soil nailing, according to the energy method, the formula of safety factor in limit equilibrium based on the log-spiral failure mode was constructed. In order to found the optimal mode to make the value of target function minimum, the author used safety factor as target function and searchedfor the dangerous slip-face, and concluded the sensitivity of all parameters to the stable factor. This paper could be used as reference in the projects.
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