Algae blooming in freshwater severely damages the aquatic ecosystem seriously, and nutrient level is the key environmental factor that results in the formation of algae blooming. For purpose of revealing the influence degrees of nutrient on the in situ growth of algae during the algae blooming, this study took the algae assemblages as the research object, and self designed devices were used in in situ experiments, which were carried out in the Pengxi River of the Three Gorges Reservoir. In this study, three different treatments including phosphorus, high nitrogen and low nitrogen treatments were introduced, and the conditions in the devices were consistent with that in the nature water body. During the phosphorus treatment as well as low nitrogen treatment, chlorophyll a showed a dramatic decreasing trend but did not manifest the same trend during the high nitrogen treatment. In order to elucidate the reason that caused the different chlorophyll a trend during different treatments, there were several methods conducted in this study to analyse and study the causes of different chlorophyll trends, such as chemometrics and correlation analysis, besides non-linear fitting. Finally, the results showed:(1) The optimal N:P for algae growth in the Pengxi River was 32.52, and the suitable phosphorus and nitrogen concentration for algae growth were 0.12 mg/L and 2.44 mg/L, respectively; (2) Both nutrient and water temperature were the key factors that resulted in the in situ algae growth. High nutrient concentration (the concentration of nitrogen was higher than 2.44 mg/L), low nutrient concentration (lower than the concentration that algae required for chemometrics), and low water temperature (lower than 20℃) might inhibit the algae growth. In contrast to the high nutrient concentration, the low nutrient concentration or the low water temperature could bring out higher inhibition for algae growth; (3) Algae blooming in the Pengxi River must meet two premises:Sufficient nutrient (higher than the lowest nutrient concentration that algae required for chemometrics) and appropriate suitable water temperature (higher than 20℃).