Based on the hydrological, water quality and water ecological monitoring data before and after the spring algal bloom in the middle and lower reaches of the Hanjiang River, the important environmental factors that have a large correlation with algae density are screened out by using Pearson correlation test, quantile regression and random forest model. The mass concentration of chlorophyll-a is simulated and predicted, and the controllable threshold of environmental factors is analyzed. The results show that the chlorophyll-a mass concentration is significantly positively and negatively correlated with air temperature and flow factors respectively, and the algae outbreak is non-linear with air temperature and flow factors. The threshold of water quality factor based on algal bloom prevention and control is that the mass concentration of chlorophyll-a is higher than 76.47.μg/L, the 7d sliding accumulated temperature is higher than 56.02.℃, and the minimum 7d average discharge is less than 780 m³/s.