With the gradual integration of large wind farms, the wind power penetration in power system is gradually increasing. Since different wind turbines access grid may have different effects on the system, it is necessary to study the effect of different wind turbines access grid on the system transient stability. This paper makes a comparative study on the system stability of constant speed wind turbine (CSWT) and the doubly-fed induction generator (DFIG) after grid-connected. Firstly, we present the dynamic models of CSWT and DFIG. Then, we use IEEE 3-machine 9-bus system to study the effect of CSWT and DFIG connected to the system on the system transient stability under different wind power penetration. Simulation results show that with the increase of wind power penetration, the transient stability of the system shows a trend of increasing first and then decreasing. However, DFIG will deteriorate the system transient stability as wind power penetration increases.