The invention discloses a water quality monitoring device and a detecting method. The water quality monitoring device comprises a shell, a front cover plate and a rear cover plate, wherein 6 hollow cavities are arranged between two end faces in the length direction of the shell. The detecting method of the water quality monitoring device comprises 7 steps of single chip microcomputer detection and the like. The water quality monitoring device successfully utilizes a TDLAS technology underwater, achieves continuous monitoring with low cost, multiple monitoring points and continuous linearity, avoids staff duty, is especially suitable for the characteristics of long lasting, continuity and mutability of hydrology and water quality monitoring and possibly has a positive effect on detecting and monitoring existing water quality environment.