Changes in seasonality of extreme precipitation have important implications for flood and fire hazards and water resources. Here we explore the contributions of extreme and non‐extreme precipitation in the sharpened wet season in California under warming and the underlying mechanisms. Modeling evidence shows that increased extreme precipitation due to enhanced extreme intensity and more extreme days dominates the wetter winter, while decreased non‐extreme precipitation due to fewer wet days induces the dryer spring and fall. Moisture budget analysis indicates that increased moisture dominates the extreme precipitation increase in winter, while weakened circulation offsets the moisture increase, resulting in no changes in spring and fall. The sharpened seasonal cycle of extreme precipitation via both dynamic and thermodynamic effects is consistent with the seasonality changes in atmospheric river days and is mainly attributed to the seasonality changes in the number of extreme days rather than the intensity during extreme days.