Abstract: The diatom blooms that are caused mainly by Cyclotella sp. occur frequently in freshwater bodies such as rivers and reservoirs, and severely damage the aquatic ecosystems. In order to study the effect of nutrient ratios on the growth of Cyclotella sp., the changes of algal cell density, Chl-a concentration and chlorophyll fluorescence parameters in the Cyclotella sp. culture were measured under different N/P, Si/P and Si/N conditions. The results showed that with the increase of N/P and Si/N, algal cell density and chlorophyll-a concentration of Cyclotella sp. significantly increased. The growth of Cyclotella sp. could be stimulated at N/P of 30 and Si/N of 5.0. When phosphorus concentration was less than 0.10 mg/L, the growth potential of Cyclotella sp. was positively correlated with Si/P, and the optimal Si/P was 100. However, when the phosphorus concentration was 0.3 mg/L, the optimal Si/P was 50. Comprehensive comparative analysis showed that Si/P had the most significant effect on Cyclotella sp. growth. With the unchanged nutrient ratio, the variation in nutrient concentration also exhibited significant effect on Cyclotella sp. growth. When the concentration of N and P was higher than 6 mg/L and 0.10 mg/L, respectively, silicon was the major limiting factor of Cyclotella sp. growth. In addition, when the P concentration was 0.03 mg/L, and N and Si were lower than 0.6 mg/L and 1.5 mg/L, Cyclotella sp. exhibited inhibited growth and limited photosynthetic activity. Therefore, our study provides important nutrient ratio thresholds, with which the outbreak of diatom blooms can be potentially controlled. The results showed that the most suitable nutrient ratios for the growth of Cyclotella sp. were N/P of 30 and Si/N of 5.0. And the optimal Si/P was related to the initial phosphorus concentration. When the phosphorus concentration is lower than 0.10 mg/L, the optimal Si/P was 100. When the phosphorus concentration was 0.30 mg/L, the optimal Si/P was 50. In general, the growth of Cyclotella sp. was most obviously affected by Si/P.