Based on the long-term numerical (1994-2018) simulation, the water exchange time scale was investigated to explore the renewal of coastal waters and its response to the climate change in the Northern South China Sea (NSCS), where the motions were driven jointly by the local terrestrial & atmospheric forcing, and remote influences from open ocean. The water exchange in the coastal exchange is largely controlled by the terrestrial processes and shows the opposite variability with the outer shelf. The intensified onshore intrusion exists over the widen shelf in NSCS, while mainly in the regions to the east of Hainan and to the southeast of the Pearl Rive Estuary, the onshore motion there has net block effect on the renewal of coastal water. Affected by the control from open ocean, the slope current facilities the transport of coastal water to the outer shelf. From coastal to the slope region, the El Niño-Southern Oscillation (ENSO) has asymmetric influence on the water renewal that coastal regions show stronger response to the El Nino year while the shelf & slope regions were mainly affected by the La Nina year.