The surrounding rock mass of a railway tunnel in southwest China is a large porosity giant loose rock pile， which is a typical discontinuous granular material and cannot be solved by classical soil mechanics based on continuous media. In order to study the strength characteristics of loose rock piles and the stability of surrounding rock from a fine-to-macro perspective， this paper starts from the mechanical analysis of granular material mechanics on tight sands， and it is obtained that the deformation mode of the plane occlusal model of loose rock piles is point-surface contact action through the analysis. The macro-strength relationship dominated by the friction angle of the rock block elements inside the rock pile is deduced. The occlusal model between the rock mass elements of the loose rock pile is proposed， and the relationship of the occlusal force of the loose rock pile is obtained. The mesoscopic friction angle between the rock block elements has a greater influence on the macroscopic strength. Starting from the Protodyakonovco theory suitable for broken surrounding rock and combining the mesoscopic strength formula of the rock pile， the calculation method of the natural balance arch axis of the loose rock pile is obtained. The surrounding rock pressure is calculated through a railway tunnel， and its rationality is verified.