Abstract: The failure of slopes composed of metamorphic schist is closely related to the combination of structural planes and slope surfaces. The block theory is outstanding in the field of analyzing the combination of structural planes and slope surfaces. The paper takes the slopes along the State Road 108 in Heihe reservoir for example and explains the method of analyzing slope stability in metamorphic schist zone with the block theory. As a preparation, the paper designs a program about the block theory and a program about the isopycnic graph of structure plane. Through the site investigation of slopes along the State Road 108 in Heihe reservoir, the slopes are divided into 15 sections and 4 of them are potential rockfall sections. This paper selects one typical point in each of the potential rockfall sections. At each point, we find out the movable block, the key block and the residual sliding force by the block theory and the self-compiling program. The combination of the structure planes of the calculated key blocks is identical to that of the fact. The results of the residual sliding force show that three of them are of high risk of rockfalls, and need engineering measures to prevent landslides from blocking the State Road 108. The paper explains the method of analyzing slope stability in metamorphic schist zone with the block theory, which has special reference to some other engineering projects.