In-situ growth rate is an important parameter for algae to study their growth, death, population variation, productivity and response to environmental changes. Currently, there is a lack of mature and accurate method to determine the in-situ growth rate of bloom-forming cyanobacteria. In this study, an improved in-situ incubation method base on the change of phycocyanin were used to investigate the annual net in-situ growth rate of bloom-forming cyanobacteria in the eastern, central and western lake areas of Lake Chaohu. The results showed that the net in-situ growth rate of bloom-forming cyanobacteria ranged from -1.16 to 0.69 d^-1. The growth rates in the surface layer were higher than those in the middle layer, and those in the bottom layer were the lowest and negative in most months. In terms of spatial distribution, the growth rates from high to low were the eastern, central and western lake areas. The growth rates were the highest in spring (April to June) and the lowest in winter (January to February). Spearman correlation analysis showed that growth rate was positively correlated with dissolved total phosphorus (DTP) and temperature, which indicates that DTP and temperature are the key environmental factors affecting the in-situ growth rate of bloom-forming cyanobacteria in Lake Chaohu.