The monitoring of phytoplankton growth state was based on chlorophyll fluorescence, mainly by using the modulated fluorometer. The changes in PSII performance of the photosynthetic apparatus have been explored by applying chlorophyll fluorescence technique. Chlorophyll fluorescence of microcystis aeruginosa and chlorella vulgaris were measured by system developed in lab. 450 nm LED were selected as excitation light source, which was used to obtain fluorescence at pigment level and light-adaptation. LD was used to supply saturation pulse which could interrupt photochemical reaction briefly. The photochemical reaction efficiency Yield was measured between phytoplankton cultivated in normal light and in control which placed in a darkroom. The maximum relative electron transport rate ETRmax tested under different light-adapted state, light use efficiency α and light damage parameter β were investigated, fluorescence induction curves and light response curves were measured, and the determination of phytoplankton growth state can be achieved.