Water serves as a key medium for the water Cherenkov detector, and the water attenuation length
signiffcantly affects the efffciency of the detectors. The water attenuation length represents the
distance traveled by 1/e of light intensity attenuation due to the absorption of photons in water,
representing the cleanliness of the water. It is a great challenge to measure or monitor the water
attenuation length accurately at home and abroad. A method, like the integrating cavity ring-down
spectroscopy is employed in this paper, which could reveal the water absorption length and the
refectivity of the material at the same time. Based on it, an automatic detection device was designed and installed. A cylindrical cavity covered with highly refective materials on the inner wall was employed to load the water. In this paper, a cover is designed which can be controlled by a stepper motor to change the depth of water in the refective chamber by moving the position of the cover, and at the same time sealing the refective chamber by using the variable air pressure of the tires. The light provided by a pulse-twinkled LED was injected into the cavity, and recorded by a fast photomultiplier tube(PMT). Finally, the collected waveforms of PMT at different water
depths of the cavity were analyzed to get the attenuation length of water and the refectivity of the inner wall. The large quantity of data also proposed a better understanding of the device
improvement.