The third-order induction motor paralleled with const impedance dynamic model is used to represent composite load, and the differential and algebraic equations (DAE) model of transient voltage stability analysis of a simple power system is established. By nonlinear dynamic system theory, the region of attraction of the post disturbance stable equilibrium point of the system is computed. In terms of the problem of determining the initial value when Newton method is used to compute the unstable equilibrium point, the method which uses steady model to represent the induction motor part of composite load is introduced. By converting nonlinear system to normal form linear system, the boundary of the attraction region of the post disturbance stable equilibrium point is computed. Then the transient voltage stability of the system can be determined just by simulating the state of the system at the fault clearing time. Compared to the simulation results of the whole fault of the system, the correctness of the method is verified.