In order to convert the weak fluorescence signals in the optical path of water optical sensor for chlorophyll a to stable electrical signals and meet the input requirements of analogtodigital conversion device, based on the early research on designing the optical path, this paper designed a signal processing circuit in optical intelligent sensor for the chlorophyll a of water. This circuit has four parts, including the excitation light source driver module, the photoelectric I/V switching and amplifier module, the filtration module and the demodulation module. For excitation light source driver module, we used the integrated driver chip which has the advantages of high bias compatibility, minimum bias and migration and lower dropout voltage to drive multiple LED at the same time. Compared with the traditional current limiting resistor project, the proposed project was more stable, highly effective and more consistent with the driven current. The photoelectric I/V switching and amplifier module which was passed through twostage amplifier called I/V conversion circuit and the same proportion amplifier circuit respectively, could amplify AC and inhibit DC compared with the traditional filter and amplification circuit. In filtration module, we used the monolithic integrated active filter without external capacitance. Compared with the traditional highorder lowpass filter, the stray capacitance’s influence was small, and the circuit size can be effectively reduced. In demodulation module, we used an integrated converter chip with incremental accumulating conversion method. Compared with the traditional peak detection circuit, the output amplitude of railrail output was wider, the conversion speed was faster and the conversion accuracy was higher (99.75%). It also had more stable output direct current and smaller temperature drift.