The integration of large-scale wind farms would influence the operation status of power system, which results in the change of transient stability characteristic, so that it is very important to quantify the impact of wind farms for ensuring the wind power using and power system security. The power system transient stability risk assessment method considering large-scale wind farms is proposed in this paper based on the research of wind power model and failure severity. Based on the consideration of the randomness of wind farm outputs and system failure states, the method employ the utility theory and take the perceived utility of operator to relative angle deviation as the measure index, then the nonlinear utility function is constructed to quantify the fault severity. Further, the calculation method and flow chart of power system transient stability index are also provided in detail. This method is applied to the New England 39-bus system, and the effect of different integration capacity and different integration point on system transient stability risk value is investigated. The simulation results show the effectiveness of the proposed method.