In this study, we utilized exposure time ((Formula presented.)) as a key metric to investigate water exchange and its spatiotemporal variations in the Northern South China Sea (NSCS). The Eulerian adjoint method and Lagrangian tracking were adopted to capture a comprehensive view of water exchange in coastal regions. Our findings reveal distinct spatial and seasonal variations in (Formula presented.). Spatially, a long (Formula presented.) (exceeding 150 days) appears in the coastal region, and the largest values occur in the Beibu Gulf (300 days). Temporally, (Formula presented.) exhibits clear seasonal patterns across the extensive shelf area, influenced by the seasonal monsoon which induced seasonally reversing shelf current and results in symmetrical distribution patterns of (Formula presented.) across the board shelf during both winter and summer months. (Formula presented.) is longer in winter than in summer. The study also revealed pronounced vertical contrasts in cross-isobath transport over the NSCS shelf, though significant vertical variations in net exchange time were noted only in specific locations, including the northeast side of Hainan Island, the Beibu Gulf mouth, and along the west side of Taiwan Island. The Beibu Gulf emerged as a critical factor in the NSCS’s water exchange dynamics in both seasons. In summer, it impacts more than 20% of the water exchange over adjacent areas, particularly through its westward transport against typical northeastward shelf currents. This highlights the combined effect of the westward spread of the Pearl River freshwater and the stable slope current on regional hydrodynamics. In winter, the Gulf’s retention characteristics profoundly affected even distant areas, contributing to up to 50% of water exchange, showing its broad impact on the NSCS’s water dynamics throughout the year.