In order to investigate the effect and mechanism of ozone on purification of the aquaculture water in the circulating water system, experiments on the effects of three different ozone dosages on the organic pollutants, ammonia nitrogen and nitrite in the water are carried out, and on this basis, the mechanism of purification is analyzed. The results show that ozone preferentially reacts with organic pollutants, followed by nitrite, and then oxidizes ammonia nitrogen; ozone selectively removes organic pollutants, preferentially reduces UV254 and water color, has no obvious effect on COD and DOC treatment, but can significantly improve the biodegradability of aquaculture water; 15mg/L ozone dosage can achieve more economical and effective purification effect;UV254, nitrite, water color and COD can be reduced by 73.86%, 92.35%, 37.52% and 16.43% respectively. DOC / UV254 can be increased to 448.86 (cm · mg) / L, significantly improving the biodegradability of aquaculture water. The research shows that the ozonation technology plays a good role in the purification treatment of aquaculture water. It is important to select the appropriate ozone dosage for different conditions to improve the efficiency of purification treatment.