In order to evaluate the effects of different plant densities on water quality and growth of fish and vegetables in aquaponics system, an aquaponics experiment was conducted in 2017. The effective volume of water for Carassius auratus gibelio breeding was 350 L, and the breeding density was 10 kg/m3. The cultivated area of Ipomoea aquatica was 1.0m2, and the density was 60, 45 and 30 plants /m2. The results showed that when the water temperature was 29.37 - 31.40 ℃, the dissolved oxygen was 2.33 ~ 5.00 mg/L, pH was 6.22 ~ 7.42 and EC was 0.19 ~ 0.66 mS/cm, the water quality could basically meet the demand for growth of fish and vegetables. The mass concentration of system ammonia nitrogen, nitrite nitrogen and nitrate nitrogen changed significantly with time (P=0.000), the mass concentration of nitrate nitrogen in different treatment was significantly different (P=0.028), and the average mass concentration of nitrate nitrogen in the system with density of 45 plants /m2 was the highest. The experiment lasted for 33d, during which the total fresh weight of fish increased by (1.70±4.36) kg, (1.50 ± 1.57) kg, (1.67 ± 6.01) kg, and the total fresh weight of vegetables increased by (7.71 ± 2.27) kg, (6.15 ± 5.49) kg, (4.04 ± 3.01) kg. The effect of plant density on the relative growth rate of nitrogen content in the aboveground part was significant (P=0.037), and the relative growth rate of nitrogen content in the aboveground part in the system with density of 45 plants /m2 was positive. Studies have shown that fish and vegetables in the systems with three plant densities grew well, the system with density of 45 plants /m2 had higher nitrogen conversion efficiency, and the nitrogen conversion of the system and more economic output could be promoted by expanding the cultivated area of plants.