Many mountainous cities are facing slope problems during project construction. At present, there are two methods to determine cracking angle for rock slope, namely, the method from "Technical Specification for Retaining Structure in Excavation" and the method from "Handbook of Engineering Geology". However, there is obvious difference between cracking angles of rock slope by these two methods. In this paper, the theoretical formulas of these two methods are derived and compared. Based on analyzing the results of FLAC numerical simulation for soft rock slope, it is concluded that the cracking angles of rock slope determined by the "Handbook of Engineering Geology" are more reasonable and safe.