Winter extreme precipitation over South China (SC) exerts profound impacts, yet its stratospheric drivers remain limited understood, with a long-standing puzzle of the statistically weak linkage between the stratospheric polar vortex (SPV) and SC rainfall extremes. Here, using observational analyses and CMIP6 historical simulations, we demonstrate that a Eurasia-shifted SPV (ESSPV) can increase SC winter extreme precipitation, but this modulation is strictly phase-locked to the Madden–Julian Oscillation (MJO), occurring predominantly during MJO phases 2–5. Two synergistic pathways explain this phenomenon. First, the ESSPV excites a Rossby wave across Eurasia, inducing a low-level northeasterly anomaly north of SC that strengthens local moisture convergence. Second, the ESSPV couples with the westerly quasi-biennial oscillation (WQBO), which amplifies the MJO-associated anomalous anticyclone over the western North Pacific and enhances low-latitude moisture transport toward SC. CMIP6 multi-model analyses further confirm that the fidelity of MJO phase 2–5-associated western North Pacific anticyclone determines a model's ability to reproduce the observed linkage, firmly establishing the MJO as the indispensable gateway linking the SPV to SC extreme precipitation.