Abstract: This paper summarizes the research achievement of D_2O purification inHWRR. The improved purification system has adopted a single mixed resin bed.The by-pass flow is increased from 0.033% to 0.1% of the main cycle flow(600m~3/h). The technology of the deuterisation, dedeterisation and transfer ofresin is improved. The efficiency of resin deterisation is approximatelly 0.1 kgD_2O per liter swelled resin. The hydraulic transfer of spent resin is completed.The volume ratio of consumed water to resin bed is 1. 1-1. 2. The result of resinpurification has shown that the specific resistance is usually over 1×10~4Ω·m, themultiple of effluent water is 2.4×10~4, and the purifucation factors of activatedcorrosion products and fission products are in the range of 10~2-10~3. Operationexperiments have shown that the purification system in HWRR can maintain thewater quality of D_2O.