During the subpit mining of surface coal mine,the reasonable establishment and removal of temporary intermediate bridge is very important to control the stability of slope. In order to solve the problem of dynamic collaborative design of slope and the intermediate bridge in multi pit parallel strip mining,and to achieve both safe production and economic benefits,the quantitative characterization method of the supporting and retaining effect of intermediate bridge and the calculation method of slope stability under this effect are studied. The intermediate bridge is regarded as a special slope retaining structure. On the basis of constructing the three dimensional mechanical model of slope and the intermediate bridge,combined with the relative position of the intermediate bridge and the slope and its own spatial shape,the intermediate bridge is divided into three zones along the slope inclination. Based on the Mohr Coulomb shear strength theory,the mathematical expression of the three dimensional supporting and retaining effect of the intermediate bridge is derived in different zones,and the quantitative relationship between the supporting and retaining effect of the intermediate bridge and its cross sectional area and volume is revealed theoretically. Starting from the essence of the three dimensional supporting and retaining effect of the intermediate bridge,by using the method of merging similar items,a twodimensional equivalent method of the three dimensional supporting and retaining effect of the intermediate bridge is proposed. The quantitative representation of equivalent cohesion cd and equivalent internal friction angle φd realized,and the influence of spatial shape parameters of the intermediate bridge on supporting and retaining effect and slope stability is expounded. The results show that the equivalent cohesion cd in zone Ⅰ is the cohesion cj of the interface at the bottom of the intermediate bridge,and there is a positive correlation between the equivalent cohesion cd in zone Ⅱ and Ⅲ and the width d of the bottom of the intermediate bridge. The equivalent internal friction coefficient tan φd of each zone is positively correlated with the bottom width d of the middle bridge and negatively correlated with the bridge height h and the cotangent value of the bottom angle α. In the engineering example,the slope stability coefficient Fs increases with the increase of the height h and bottom angle α of the intermediate bridge,and the rising gradient decreases gradually and has a positive correlation with the bottom width d and the bridge length b. The results can be applied to the dynamic optimization design of morphological parameters of the intermediate bridge in an open pit coal mine and the treatment of slope stability under the action of intermediate bridge support.