Under weak grid conditions, there is a strong nonlinear coupling between phase-locked loops (PLLs) and the doubly-fed induction generators (DFIGs), and the influence of its dynamic characteristics on system stability cannot be ignored. For a system to be stable, the dynamic properties must be considered. First, based on instantaneous power theory, the subsynchronous frequency component of power oscillation in a three-phase symmetrical system with the participation of a PLL is deduced. Second, the PLL transient stability is studied under two power grid structures. Third, to connect with the power grid model, a DFIG with a PLL structure is established, and the phase disturbance of the PLL is used as the excitation signal. In the rotor current inner loop and the voltage outer loop transfer processes, the PLL phase disturbance component plays a role. A d-axis structure block diagram of the current loop with the PLL phase disturbance component is constructed, and the disturbance transfer function is deduced. The influence mechanism of the PLL control parameters on system power oscillation is investigated. Finally, the modeling simulation verifies the validity of the theoretical analysis