According to the phytoplankton fluorescence induction characteristics under different light conditions, chlorophyll fluorescence as a probe for analysis of phytoplankton photosynthesis was studied. The present paper proposed a in-situ measurement method based on the chlorophyll fluorescence values Ft and Fm to get phytoplankton photosynthesis activity, Chlorella vulgaris, microcystis aeruginosa and Cyclotella meneghiniana Kiits were selected as experimental subjects, a comparison test was done between self-developed in-situ measurement system and Water PAM in lab, and The results showed that coefficients between the two methods were 0.977 8, 0.878 6 and 0.795 3. This work provides a rapid and in-situ measurement method for phytoplankton photosynthesis activity.