As a result of industrial activities, aquatic ecosystems have been contaminated increasingly by metals. Such occurrences pose a threat to aquatic organisms in particular and to the whole ecosystem in general. Because of their importance as a part of the food chains in the freshwater ecosystems, as well as their high vulnerability to metal contaminants, rotifers have attracted attention by toxicologists. Zinc is essential for life at trace level, but it involved in the endocrine processes and reproduction of mammals. As one of the major metal contaminants in freshwater ecosystems, zinc is of ecotoxicological interest. In order to investigate the chronic toxicity of Zn²⁺ to organisms in an aquatic environment under different food densities, and to screen out sensitive endpoints for monitoring Zn²⁺ pollution with rotifers as test animals, this paper studied the effects of Zn²⁺ concentrations (0.1, 0.3, 0.5, 0.7 and 0.9 mg/L) on the population growth parameters of Bracionus calyciflorus under different Scenedesmus obliquus densities (1.0×10⁶, 2.0×10⁶ and 4.0×10⁶ cells/mL). The results showed that at 25℃, the 24 h LC₅₀ of Zn²⁺ to B. calyciflorus under 1.0×10⁶, 2.0×10⁶ and 4.0×10⁶ cells/mL of Scenedesmus obliquus was 6.647, 8.102 and 5.873 mg/L, respectively. Compared with the controls under the same food density, when S. obliquus density was 1.0×10⁶ cells/mL, all the concentrations of Zn²⁺ had no significant effects on all the population growth parameters of B. calyciflorus (P > 0.05). When S. obliquus density was 2.0×10⁶ cells/mL, Zn²⁺ at all the test concentrations significantly prolonged the life expectancy at hatching, the generation time and the average lifespan, and increased the net reproductive rate, but Zn²⁺ at all the test concentrations except 0.3 mg/L decreased the intrinsic rate of population increase of B. calyciflorus. When S. obliquus density was 4.0×10⁶ cells/mL, Zn²⁺ at 0.1, 0.3 and 0.7 mg/L increased the intrinsic rate of population increase, Zn²⁺ at 0.7 and 0.9 mg/L increased the proportion of sexual offspring of B. calyciflorus. S.obliquus density had significant effects on the life expectancy at hatching, the generation time, the net reproduction rate, the intrinsic rate of population increase, the average lifespan and the proportion of sexual offspring (P < 0.01), Zn²⁺ concentration had significant effects on the life expectancy at hatching, the generation time, the net reproduction rate, the intrinsic rate of increase and the proportion of sexual offspring (P < 0.01), and the interaction between S. obliquus density and Zn²⁺ concentration had significant effects on the life expectancy at hatching, the intrinsic rate of increase and the proportion of sexual offspring of B. calyciflorus (P < 0.05). When S. obliquus density was 2.0×10⁶ cells/mL, there were significant dose-effect relationships between Zn²⁺ concentration and the life expectancy at hatching, the generation time, the net reproductive rate as well as the average lifespan, and the relationships could be described as y=-73.949x²+110.190x+109.573, y=-37.568x²+47.452x+78.233, y=-18.799x²+23.226x+14.908 and y=-45.602x²+54.730x+66.303, respectively. When S. obliquus density was 4.0×10⁶ cells/mL, there was a significant dose-effect relationship between Zn²⁺ concentration and the proportion of sexual offspring, and the relationship could be described as y= -9.648x²-1.978x+22.222.