According to the problems in the detection of weak chlorophyll fluorescence, a chlorophyll fluorescence detection system based on m sequence modulation technology was proposed. On the basis of noise analysis of fluorescence detection circuit, I/V conversion circuit and secondary amplifier circuit that based on T-type resistance feedback network were designed. According to the principle of m sequence demodulation, a bipolar conversion circuit, a phase-shift circuit and a fluorescence signal demodulation circuit based on AD630 were designed. The system was applied to measure the chlorophyll fluorescence of chlorella pyrenoidosa. For a sample with a chlorophyll concentration of 10 μg/L, the relative standard deviation of measurement values was 2.61%. The linear correlation coefficient between the output DC voltage and chlorophyll concentration was 0.998 in a chlorophyll concentration range of 0～100 μg/L.