Abstract: The stability of long distance transmission system with TCSC is analyzed in this paper, and the electromagnetic power of generators at different fault points is deduced by using of a simplified model. Then the inner relation between parameters of power grid, compensation degree of TCSC and transient stability of the transmission system is analyzed, and the influence of faults that occur at different points on system stability is studied, which will determine the fault point that restricts the transmission capability. In the end, this model is used to find out the fault point that restricts the power output capability of Yimin Power Plant in Northeast Power Grid, which verifies the validity of the model.