Antibiotics are commonly used in human medicine and animal husbandry for the prevention and treatment of illnesses and the promotion of growth. Due to high consumption, incomplete metabolism and continuous discharge, antibiotics are frequently detected in the aquatic environment. Resistant bacteria and genes caused by antibiotics are receiving considerable attention, but the sublethal effects of antibiotics on the non-target organisms, especially the aquatic organisms, should be taken into consideration, due to their important role in aquatic ecosystems and their potential sensitivity to contaminants. In order to understand the chronic toxicity of antibiotics to aquatic organisms under different algal food densities, the freshwater rotifer Brachinionus calyciflorus was used as test animal to investigate the effects of penicillin sodium concentrations (0, 200, 400, 600, 800 and 1000 μg/L) on its life-table demographic parameters under three levels (1.0×10⁶, 2.0×10⁶ and 4.0×10⁶ cells/mL) of Scenedesmus obliquus. The results showed that compared to the control under each algal density, and when the algal density was 2.0×10⁶ cells/mL, treatments with penicillin sodium at 200-1000 μg/L prolonged life expectancy at hatching and generation time by 40.67%-70.67% and 34.04%-50.23%, and increased net reproductive rate and intrinsic rate of population increase of the rotifers by 204.35%-358.70% and 36.26%-62.09%, respectively (P<0.05). Both increase and decrease in algal density decreased the magnitudes of promoting effects of penicillin sodium on survival, reproduction and population growth of the rotifers. At 1.0×10⁶ cells/mL of S. obliquus, treatment with penicillin sodium at 400 μg/L prolonged the life expectancy at hatching by 21%, treatments with penicillin sodium at 200-400 μg/L prolonged the generation time by 23.15%-33.13%, and those at 200-600 and 1000 μg/L increased the net reproductive rate by 40%-81.05% and 41.05%, respectively (P<0.05). At 4.0×10⁶ cells/mL of S. obliquus, however, penicillin sodium concentration did not affect any life-table demographic parameters of the rotifers (P>0.05). In the range of tested penicillin sodium concentrations, and at 1.0×10⁶ cells/mL of S. obliquus, significant effect-dose relationships were observed between life expectancy at hatching, generation time as well as net reproductive rate and penicillin sodium concentration, and could be described as y=-6.07×10^-5x² +0.098x+ 62.629, y=-4.10×10^-5x²+ 0.041x+52.55, and y=-2.640×10^-5x²+ 0.042x+ 5.238, respectively. At 2.0×10⁶ cells/mL of S. obliquus, significant effect-dose relationships were observed between life expectancy at hatching, generation time as well as intrinsic rate of population increase and penicillin sodium concentration, and could be described as y=-6.187x² +0.098x+ 51.464, y=-3.821×10^-5x² +0.054x+ 45.679, and y=-4.389×10^-8x²+6.417×10^-5x+ 0.037, respectively. The present results indicated that penicillin sodium at environmental relevant concentrations would not significantly affect the survival, reproduction and population growth of rotifers.