<p>The impacts of the quasi-biweekly oscillation (QBWO) on the diurnal variation of rainfall over South China during the pre-summer rainy season are examined using satellite observations and ERA5 reanalysis data. Rainfall is suppressed in the QBWO phases 7, 8 and 1 while enhanced in phases 3, 4 and 5. During phases 7, 8 and 1, the background southwesterly winds are suppressed and the western Pacific subtropical high (WPSH) retreats eastward. The nocturnal to early-morning rainfall anomalies in the suppressed phases are more pronounced due to the inertial oscillation mechanism. During phases 3 to 5, the southwesterly winds are strengthened and the WPSH extends westward. In phase 3, rainfall exhibits an evening anomalous rainfall peak, which is closely related to the effects of strong low-level onshore winds. Such a strong low-level wind is caused by the combined effects of both the westward extension of the WPSH and the downward momentum propagation originating from the tropical region. During phases 4 and 5, the anomalous rainfall peaks in the afternoon due to the increased convective instability. These findings highlight the modulation of the QBWO on the diurnal cycle of rainfall primarily by regulating the intensity of low-level monsoonal wind and its associated moisture transport, providing valuable insights for improving sub-seasonal to diurnal forecasts and enhancing the prediction of the pre-summer extreme rainfall events over South China.</p>