This paper takes the large geotechnical sand bag as the research object，and builds the mathematical equation based on the force analysis. The paper discusses the influence of friction coefficient of sand bag and lateral pressure coefficient of the filling on the deformation of large geotechnical sand bag，then analyzes mechanism of the two factors，and verifies the results with the design and selection of sand bag in an actual project. The results show that the tension of the bag increases and bulges continuously with the increase of friction coefficient. The tension of the bag increases with lateral pressure coefficient changes but the shape is nearly stable. The friction coefficient of sand bag has more obvious effect than lateral pressure coefficient of filling. The built mathematical model is basically reliable，and has excellent practical value.