There is a risk of inducing subsynchronous oscillations in the mode interactions between wind power and the thermal units that are bundled with it. In this paper, the law of mode quasi-strong interaction of a wind-thermal bundled system is studied: with a change of parameters, the permanent magnetic synchronous generator (PMSG) phase-locked loop (PLL) mode is close to that of the thermal power shaft system and a mode quasi-strong interaction occurs. The motion direction of two modes changes, whereby the weakly damped mode crosses the imaginary axis and quickly becomes the negatively damped mode, triggering subsynchronous oscillation, and the system oscillation disappears after the modes move away from each other. It is shown that both the turbine shaft mode of thermal power plants and the PLL mode may cross the imaginary axis leading to subsynchronous oscillations, while the main part of the system oscillations is determined by the negatively damped modes. Enhancing the damping of the modes where coupling occurs helps to suppress the interaction, and increasing the number of grid-connected turbines and line impedance both enhance the risk of system subsynchronous oscillations and exacerbate the degree of oscillations. Finally, the correctness of the theoretical analysis is verified by building a model of a wind-thermal bundled system based on PSCAD/EMTDC, and countermeasures are proposed to avoid the occurrence of oscillations.