This study reports the results of geochemical behavior of particle-reactive radionuclides, 234Th, 210Pb, and 210Po in Dongsha coral reef system. The sampling was carried out during June 2013 - September 2014 in the lagoon water. Temporal variation and partitioning in different particle sizes of the three radionuclides were investigated by a sampling scheme of filtration and size fractioning. The results showed that 234Th, 210Pb, and 210Po tend to concentrate in particles of the size ranging from 0.45 to 10 µm. It seems that 234Th also concentrates in particles of 153-335 µm, indicating the effect of colloid aggregation and/or the dominance of fecal pellets. The distribution coefficient (Kd), an indicator of particle affinity of elements, of 234Th, 210Pb, and 210Po are 102.7-106.5, 104.0-105.3 and 104.5-105.3 mL/g, respectively, which suggests the binding order of 210Po > 210Pb > 234Th in the lagoon. The 210Po/210Pb ratio in the particles ranged from 0.12 to 2.06, suggesting that the particulate pool might mainly be composed of inorganic materials. In addition, the 210Po/210Pb ratio decreases with particle size within the size range of 10-335 µm. Estimated by the irreversible scavenging model, the residence times of 234Th (tTh) and 210Po (tPo) are < 1 day and 16 day, respectively, similar to the values reported in coastal waters. The total 210Po activity in the lagoon was 8 to 10 fold higher than that in the vicinity of the South China Sea, which may be the result of accumulation of atmospheric deposition of 210Pb in the semi-closed atoll. Moreover, daily variation of 234Th, 210Pb and 210Po in the particulate phase is caused by resuspension of the underlying sediments in phase of tidal wave, which is consistent with the daily variation of particle concentration.