Aiming at the problem of small disturbance stability of grid-connected doubly-fed wind farms dynamically dominated by phase-locked loop, a method for determining small disturbance instability of doubly-fed wind farms is proposed. Firstly, a second-order linearization model of a single doubly-fed wind turbine is established. Secondly, the derived second-order linearization model is extended to grid-connected doubly-fed wind farms based on the single-machine equivalence model. Finally, the small disturbance stability limit of grid-connected doubly-fed wind farms dynamically dominated by phase-locked loop is derived based on the Rausch-Hurwitz theorem which explains the influence law and the intrinsic mechanism of active output of wind turbines, grid connection strength, phase-locked loop control parameters and the number of wind turbines on the small disturbance stability of grid-connected doubly-fed wind farms dynamically dominated by phase-locked loop. The results show that the small disturbance stability of the system is adversely affected by the increase of active output, the weakening of system connection strength and the inappropriate setting of the control parameters. A grid-connected doubly-fed wind farm consisted of 15 doubly-fed wind turbines is used to verify the correctness and effectiveness of the small disturbance stability limit proposed, and it is not necessary to set up a full-order model of the wind farm, which reduces the amount of calculation.