At present the AC/DC transmission system with wind-thermal combined generation is a new way to transmit power，and its characteristics of stability should be paid attention to.Based on extend equal area criterion（EEAC）theory，the impact of the wind penetration on angle transient stability was studied.The function of angle acceleration changing with the wind penetration was deduced with the equivalence of wind farms' mechanical and electrical output power to the increment of critical group's mechanical power.And the conditions of satisfying the function to be a convex one were also proposed.If the equivalent inertia of the remaining group could be treated as infinite comparing with that of the critical group，the first swing stability of the relative angle of the two groups is improved as the wind power penetration increases.On the contrary，it would be improved firstly and then aggravated according to the wind power penetration's increment.The results of simulation show that the analysis of paper is correct.