  Water Cherenov detector is a vital part in most of neutrino or cosmic ray
research. As detectors grow in size, the water attenuation length (WAL) becomes
increasingly essential for detector performance. It is essential to measure or
monitor the WAL. While many experiments have measured WAL in the lab or
detector, only the Super-Kamiokande experiment has achieved values exceeding 50
meters in the detector with a moving light source. However, it is impractical
for many experiments to place a moving light source inside the detector,
necessitating an alternative method for investigating long WAL. A novel system
has been proposed to address the challenge of investigating long WAL. This
system focuses on ample water Cherenkov detectors and features a fixed light
source and photomultiplier tubes (PMTs) at varying distances, eliminating the
need for moving parts. The static setup demands high precision for accurate
measurement of long WAL. Each component, including LED, diffuse ball, PMTs, and
fibers, is introduced to explain uncertainty control. Based on lab tests, the
system's uncertainty has been controlled within 5\%. Additionally, camera
technology is also used during the evaluation of the system uncertainty, which
has the potential to replace PMTs in the future for this measurement. Monte
Carlo simulations have shown that the system can achieve a 5\% measurement
uncertainty at WAL of 80 meters and 8\% at WAL of 100 meters. This system can
be used in experiments with large Cherenkov detectors such as JUNO water veto
and Hyper-K.
