Considering the interaction between generators, the dynamic energy model of direct-drive wind farm is proposed, and the effect mechanism of energy interaction on sub-synchronous oscillation is revealed. First, the dynamic energy model of direct-drive wind farm containing the generator-grid interaction links, inter-generator coupling interaction links and inter-generator induction interaction links is constructed. These energy links reflect the energy interaction between the generator and the grid, and between the generators during oscillation. Then, the dissipation energy representing the damping characteristics is extracted. Thus, the contribution degree of each interaction is quantitatively evaluated. On this basis, the influences of the current loop parameters, PLL parameters, the integration distance and the reference value of active current on the stability level are revealed, and the effect mechanism of generator-grid interaction and inter-generator interaction on sub-synchronous oscillation is clarified. Finally, a model of direct-drive wind farm is built, and the simulations show that, the generator-grid interaction and inter-generator coupling interaction are the key factors that affect the energy distribution. As the proportional gain of current loop and phase-locked loop and integration distance change, the two interactions are mutually exclusive.