This paper presents a method about quantitation and classification of transient stability risk assessment based on the risk theory. From the point of risk theory, Uncertain factors include the operation, the fault occurring, the fault type, the fault location and the automatic reclosing process and so on, the consequence is the percentage of load cutting caused by transient instability, and then the risk value of transient stability is derived which is quantitative, then use the ALARP principle based on "safety accident investigation procedures of State Grid " to quantify and classify the risk value, which can make the meaning of risk value more clear, and also provide a reference for operating personnel. Finally, the WSCC 3 unit 9 bus test system is used to illustrate the feasibility and effectiveness of this method.