Wind turbine with direct-drive permanent magnet synchronous generator (DDPMSG) can provide frequency support for power system under the control of virtual inertia, but it creates new coupling relationship between the DDPMSG and the system, which affects the small signal stability of the power system. This paper sets forth a state space averaging model of wind power integrated system containing the DDPMSG by reasonable simplification, and obtains its small signal model through linearization. Based on the small signal model, it analyzes the impact of proportion coefficient and differential coefficient of the virtual inertia control on small signal stability of the wind farm integrated system, and comparatively analyzes the impact of the virtual inertia control and maximum power point tracking control on small signal stability of the power system, by the eigenvalue analysis method. Results show that capacity and location of the wind power, controlled by virtual inertia, make a difference in damping performance of power system, and mainly in the integrated area’s local oscillation mode and inter-area oscillation mode.