The role of ecological bags in the slope stability analysis and the minimum size of ecological bags to meet the local stability under the water level fluctuation zone are discussed by a calculating example. The safety analysis method of ecological bags is given. The results show that ecological bags can improve the stability of slope and avoid the shallow sliding of surface layer. The safety factor of slope stability increases with the increase of the thickness of ecological bag protection range. The size of ecological bag increases exponentially with the increase of river velocity and is sensitive to the river velocity. The computational formulas of anti-slip analysis, anti-overturning analysis and anti-shear analysis are simple and intuitionistic, but the values of technical parameter and safety factor are the key.