Measurement of chlorophyll fluorescence of phytoplankton provides real-time information on their physiological status. The determination of the physiological state of phytoplankton is of vital significance to the fields of natural disasters and water ecology research. In this study, for the detection of chlorophyll fluorescence in phytoplankton photosynthesis in seawater, a weak signal detection circuit and a high-speed data acquisition circuit were designed, and the cross-group amplification and conversion of weak photocurrent signals, as well as the design of adjustable bias second-stage amplification circuits were implemented. Actual measurements were carried out on samples from the Chlorophyta (Platymonas), Chrysophyta, Cyanobacteria, and Pyrrophyta (Prorocentrum donghaiense Lu). The results show that the designed circuit has a minimum detection current of 80 nA under excitation from a 500 kHz light source, a detection range of 80 nA-100 μA, and achieves a sampling frequency of 1 MHz. In the application of the phytoplankton chlorophyll weak fluorescence detection device, the circuit ensures the sensitivity of the system. It extends the detection range of weak signals, providing important data support for research related to chlorophyll fluorescence detection.