Absorbance is an important parameter to describe the optical properties of sea water, it can directly reflect the transparency of sea water and the degree of light attenuation of sea water, it is one of the inherent optical properties of water. The results of absorbance of sea water have an important guiding effect on the field of underwater optical communication and spectral detection. Too little spectral information can be obtained by the existing absorbance measurement equipment, hyperspectral absorbance measurement equipment has problems such as expensive price and large volume, so it is not suitable for large range of in situ absorbance detection. In order to achieve a wide range of long-term measurement of sea water absorbance, an in-situ absorbance sensor suitable for ocean towed observation systems is designed. The sensor can realize the synchronous detection of multi-band absorbance, select LED as a light source of the sensor, and select photodiode as detector respectively of the sensor, the optical path of the sensor absorption cell is 10 mm. The transmission optical path structure design is adopted, the multi band LED is located at the transmitter end of the sensor, and the detector is located at the receiving end of the sensor. And realize the miniaturization and low-power consumption design of the sensor through the multi-channel side-by-side arrangement of the photomechanical structure layout and highly integrated circuit optimization. The sensor selected 8 LED light sources of relatively important feature bands at 340~980 nm for absorbance measurement. The 8-channel LEDs at the transmitting end are modulated with 781.25 Hz sine wave signals, after the beam is collimated, it is attenuated through the absorption cell, and then converged to the photodetector at the receiving end. After receiving the signal, the photodetector demodulates and analyzes the signal using a digital phase-locked algorithm based on the cross-correlation detection principle to achieve absorbance measurement. From the optical perspective, the reference detector was placed near the LED light source to calibrate the influence of light source fluctuation on the absorbance result. The sensor used a narrow-band filter with a high cut-off depth to filter out stray light outside the band at the receiving end. At the same time, it can avoid the influence of scattered light from adjacent channels. From the circuit point of view, the signal received by the detector is realized by a digital phase locking algorithm. The narrow-band optical filter and digital phase-locked processing algorithm at the receiving end can effectively suppress the influence of background stray-light and signal interference noise, and realize in situ detection of seawater absorbance in the open field environment, which is the key to achieve high precision absorbance measurement. The laboratory precision test and the comparison test with the same type of equipment and the South China Sea test prove that the absorbance sensor has the advantages of high precision, small volume, strong anti-environmental interference ability, low power consumption and stability, and the precision of the sensor is better than 0.000 1 AU (Absorbance Unit). In addition, the absorbance sensor uses photodiode as the detector to make the whole system have a fast response speed, which can capture the rapid change of the absorbance of the marine environment, the absorbance sensor can achieve different detection requirements by replacing the light source of other bands and the corresponding filter. During the use of the sensor, temperature and salinity sensors are also used to adjust the temperature and salinity of the absorbance results, so as to obtain more accurate measurement data. The absorbance sensor can also adapt to the complex water environment by designing circulation cell of different path lengths. It can also carry ships in coastal waters to achieve continuous detection by sailing, which has a wide range of application prospects.