Abstract: In order to explore the water purification effect of paddy field in the combined pond-paddy field farming system, we analyzed the biofiltration ability of sea rice paddy field in two shrimp culture seasons (Summer and winter). In the summer trial, the tail water of shrimp culture was purified by sea rice and common rice paddy fields with different planting densities, and the purification efficiency of sea rice paddy fields with standard planting density was the highest. After six weeks, the removal rates of ammonia nitrogen, nitrate nitrogen, nitrite nitrogen, total phosphorus and chemical oxygen demand in standard sea rice treatment were 81.6%, 68.2%, 63.7%, 91.0% and 29.7%, respectively, which were significantly higher than those in the control treatment (P<0.05), and the removal rates of ammonia nitrogen and total inorganic nitrogen were significantly higher than those in the common rice treatment (P<0.05). In the winter trial, sea rice was harvested in the shrimp-rice pond, the average concentrations of ammonia nitrogen, nitrite nitrogen, total inorganic nitrogen and total particulate matter in the shrimp-rice ponds decreased by 51.5%, 40%, 36.7% and 11.2%, respectively, compared with the shrimp monoculture ponds (P<0.05). The average concentrations of nitrate nitrogen, total phosphorus and chemical oxygen demand, particulate organic matter and particulate inorganic matter were not significantly different from those in monoculture ponds (P>0.05). The results show that the sea rice paddy field has certain purification ability in both rice growing season and non-growing season, which helps to maintain a good water environment for shrimp growth.