Abstract: Radium isotopes were important tracers for water transport and mixing processes. We investigated the distribution of ²²⁶Ra and ²²⁸Ra in the northern slope of the South China Sea (SCS) during spring based on a cruise conducted in March 2017. The results proved that the activities of ²²⁶Ra in surface water varied from 6.4 to 10.7 dpm/100 L, and those of ²²⁸Ra varied from 4.6 to 26.0 dpm/100 L. The ²²⁶Ra and ²²⁸Ra activities, temperature and salinity in the surface seawater reflected that the surface water in the northern SCS was mainly composed of two water masses, namely the SCS water and the Kuroshio branch water. The proportion ranges of the two water masses in study area was 0.13～1.05 and −0.05～ 0.87, respectively. The activity ranges of ²²⁶Ra and ²²⁸Ra in sea water column were 5.9～29.7 dpm/100 L and 2.6～6.5 dpm/100 L. Using a steady-state hypothesis and the two-box model of ²²⁶Ra and ²²⁸Ra, the residence time of seawater below 1500 m in the northern SCS was estimated to be approximately 14～61 years.