The invention relates to an online water quality monitoring device and method based on full spectra. The device comprises a pulse xenon lamp, a cleaning module, a spectrum measuring module and a control unit. One end of the cleaning module is connected with the pulse xenon lamp, the other end of the cleaning module is connected with the spectrum measuring module, the control unit controls the pulse xenon lamp to emit pulse light, controls the cleaning module to clean a measuring surface as needed and carry out light path switching, and controls the spectrum measuring module to read the light signal intensity of each waveband. According to the online water quality monitoring device and method based on full spectra, ultraviolet visible full waveband absorbing spectra are adopted for measuring the water pollution condition, factors such as COD, BOD, TOC, nitrate, nitrite, the turbidity and chromaticity can be measured, and the online water quality monitoring device and method based on full spectra have the advantages of being high in measuring precision, short in measuring time, large in number of monitoring factors, free of a reagent and secondary pollution and the like; the changing condition of water pollution is reflected in real time, and corresponding chemical models can be established according to ingredients of field water samples to meet the demands for measuring different water bodies.