Abstract: The Water Cherenkov Detector Array (WCDA) is one of the main detectors of the High Altitude Cosmic Ray Observatory (LHAASO), with water as the only detection medium, the water transparency will directly affect the detection efficiency of Cherenkov Light. In order to ensure the realization of WCDA physical goals, the real-time measurement and monitoring of water attenuation length is very vital, which is one of the key tasks for the normal operation and calibration of the detector. In this experiment, the working principle of water attenuation length measuring device and ultraviolet-visible spectrophotometer is introduced, and various samples of water are measured by different wavelength LED, and the data results of the two methods are compared and analyzed. The measurement errors of the two devices are found to be 0.22m and 0.18m, the corresponding relationship between industrial index absorbance and water attenuation length of scientific research index is found, and a Querry water quality model for the GEANT4 simulation is established by simulation, which further promotes the realisation of simulation. Through the verification of WCDA No.1 water attenuation measuring device and the data study of water quality monitoring from water injection to stable operation, a set of stable and reliable water quality monitoring scheme is summarized, which provides a lot of help for the monitoring work of No.2 and No.3 water pools.