The sensor of chlorophyll a based on fluorimetry was designed. It can satisfy the long-term and real-time requirement of sea and lake detection. Firstly, this paper introduces the basic principle of fluorimetry’s detecting chlorophyll a based on the former research. The fluorimetry sensor belongs to optical sensor. Optical path design, the key part of hardware, was described as follows: first of all, the optical sensor probe was designed; then the 460nm super bright blue light diode was selected as the fluorescence excitation signal source, because chlorophyll a has the strongest absorption in 430~470nm blue brand. Four sets of super bright blue light emitting LED were used, which suited the characteristic of chlorophyll a’s absorption and had the advantages of low power consumption, small volume, low cost and simple circuit design. For the photosensitive receiver module, there are advantages and disadvantages of different electric converters. Finally, the light-sensitive diode was chosen, as it satisfied the requirement of weak fluorescent signal of chlorophyll a. The sensor was calibrated at last. This paper built the standard curve of sensor and the standard curve based on the direct fluorescent method, the correlation coefficient of sensor meansurement is 0.998, standard deviation is 0.05μg, and limit of detection is 0.25μg/L. 