In order to accurately assess the effect of wind farms on the available transfer capability (ATC), probabilistic available transfer capability of wind farm incorporated system is researched, and the change law of ATC with different wind farm incorporated system is analyzed in detail. First, the mathematical model of large scale wind farm is established based on the Weibull model of wind speed. Then, primal-dual interior point method is used to calculate the value of ATC in certain system operation mode. Finally, ATC probability assessment of wind power integrated system is achieved by Monte-carlo simulation method in a broad perspective. The simulation results show that the proposed algorithm can effectively assess the influence of wind farms to ATC, which has guiding functions for future expansion of the power grid and can provide effective reference information for the safety and economics evaluation of the system with wind farms.