Flexible AC Transmission System(FACTS) devices can improve the transient stability of power system effectively，so the risk of system instability is reduced. After analyzing the existing transient stability analysis method

the control strategy of three typical FACTS devices- Thyristor Controlled Series Compensator(TCSC)

Static Var Compensators(SVC) and Unified power flow controller(UPFC) for transient stability is proposed . Based on probabilistic risk assessment theory

the Monte-Carlo simulation is used to assessment transient instability risk of power system with and without FACTS devices with considering several random factors

then the method to select the place where FACTS devices installed and the comparison of the effect of different FACTS devices based on generator curtailment risk are proposed. At last the effect of several random factors on FACTS devices affection is analyzed. Simulation results of RTS79 system show the effectiveness of proposed model and algorithm.