Diurnal (S1) and semidiurnal (S2) oscillations are major factors in determining the sub-daily variations of precipitation amount over Southeast China (PSEC). Most studies have examined the causes of S1(P)SEC and S2(P)SEC in the summer rainy season. However, causes of the seasonal changes in S1(P)SEC and S2(P)SEC have not been well documented. This study therefore examines possible mechanisms behind various precipitation types/cloud types because different types of precipitation/clouds control PSEC in different seasons. Results indicate that the variation of S1(P)SEC in winter, which tends to peak in the early morning, is mainly controlled by the high-relative-humidity-induced diurnal variation of non-showery precipitation/middle-level clouds. For S1(P)SEC in the other seasons, which tends to peak at the late afternoon, the moist-convection-induced diurnal variation of showery precipitation/low-level clouds is the main cause. Analyses also suggest that the phase of S2(P)SEC does not vary seasonally because both of its formation mechanisms—the semidiurnal variation of relative humidity and moist convection process—have similar phase evolution in all seasons. Seasonal changes in the water vapor supply to the maintenance of S1(P)SEC and S2(P)SEC are also discussed.