With the integration of large-scale direct-drive wind farms into weak AC grids, the complex interactions between the two make the short-term voltage stability problem of the system after perturbation more and more prominent. How to accurately analyze the short-term voltage stability of the grid-connected system of direct-drive wind farms after disturbances online is a key challenge. To this end, a short-term voltage stability analysis method based on the generalized instantaneous energy function method is proposed. First, a dynamic model of the grid-connected system of direct-drive wind farms in the phase-locked loop dq coordinate system is established. Then, the generalized instantaneous energy function of the grid-connected system of direct-drive wind farm is constructed according to the principle of reactive power conservation. Based on the generalized instantaneous energy function, dynamic reactive power energy security regions (DRPESR) are proposed to evaluate the short-term voltage stability of the system online. A hierarchical energy structure is constructed to analyze the short-term voltage destabilization mechanism of the grid-connected system of direct-drive wind farms. Finally, the time-domain simulation results based on Matlab/Simulink demonstrate the validity of the generalized instantaneous energy function and the accuracy of the proposed criterion.