Abstract Tropical cyclone (TC)‐induced extreme precipitation (TCEP) poses heavy risks, yet its seasonal predictability remains limited by insufficient understanding of cross‐scale climatic drivers. Here, we elucidate a stratosphere‐to‐stratosphere coupling pathway where the quasi‐biennial oscillation (QBO) modulates TC activity and subsequent TCEP over the southeastern coast of China, with the second boreal summer intraseasonal oscillation mode (BSISO2) acting as a critical mediator. The westerly QBO phase reshapes the spatial structure of BSISO2 during its Phases 4–6, creating favorable dynamical conditions and moisture convergence that nearly double TCEP over the southeastern China coast relative to the easterly QBO phase. Statistical fitting results further show that incorporating the QBO signal substantially improves seasonal TCEP predictability beyond conventional El Niño‐Southern Oscillation predictors. These findings establish the QBO as a coherent stratospheric driver for TCEP, providing a mechanistic basis for enhanced seasonal TCEP prediction and regional disaster preparedness.