The stability of grid-connected operation of full-scale wind turbines in wind farms is often related to the control of the grid-side converter of the turbine and the impedance between grid and the turbine. Therefore, the instability of wind turbines in a wind farm does not always occur at the same time. In order to reveal the relationship between the location of wind turbines and their transient stability, as well as the interaction on stability between wind turbines, a nonlinear reduced order model of multi wind turbines based on phase-locked loop is established. Based on the theory of equal-area-principle, the transient stability margin of different turbines in the wind farm under different operating conditions and the influence of line impedance on the stability are then analyzed. Through Matlab/Simulink time domain simulation, quantitative analysis and verification are carried out. The results show that the feeder impedance in the wind farm will reduce the transient stability region of the connected turbines whose output current flow through the impedance, and has little effect on the transient stability of other turbines whose output current does not flow through the impedance. The greater the current flowing through the impedance is, the smaller the stability margin. Therefore, with the increase of the number of turbines linked on a feeder line, the impact of the internal impedance of a wind farm on stability cannot be simply ignored. In addition, the larger output of a wind turbine, the easier it is for it to become unstable under the same conditions. This work is supported by the Key Project of the Natural Science Foundation of China (No. 51837007).