To investigate the impact of light intensity on the growth and photosynthesis activity of Pseudanabaena sp., which is a common species in freshwater cyanobacteria blooms, five light intensity gradients were designed according to the natural light intensity range. Experiments were conducted by pure culture and lasted for 16 days, during which biomass, chlorophyll fluorescence parameters, and the pigment composition of Pseudanabaena sp. were tracked. Results indicated that the optimum light intensity for Pseudanabaena sp. growth was 1 500 lx during 500-2 500 lx. The sequence of the special growth rate of Pseudanabaena sp. from high to low was 1 500 lx, 1 000 lx, 2 000 lx, 2 500 lx, and 500 lx. The photosynthesis activity of Pseudanabaena sp. was significantly correlated with light intensity. After a period of culturing, the photosynthetic rate and the light use efficiency of Pseudanabaena sp. had obvious negative correlation with light intensity, and on the 11th day the correlation coefficients were -0.939 (P<0.05) and -0.978 (P<0.01), respectively. Half light saturation had significant correlation with light intensity, and the coefficient was 0.976 (P>0.01) on the 11th day. Pseudanabaena sp. belongs to low light intensity tolerant cyanobacteria and has self-regulation mechanism that can deal with insufficient or excessive light. It requires relatively low light intensity compared with other algae species and is easy to dominate in water with low or fluctuant light intensity. Outbreaks of Pseudanabaena sp. are relatively unrestricted by water depth and light intensity, so it is not suitable to use vertical disturbance and shading technology for the control of Pseudanabaena sp. Results of this study can contribute to the prediction and control of Pseudanabaena sp. in surface waters.