A high resolution wide-spectral micro-spectrophotometer is developed based on the asymmetrical cross Czerny-Turner system. The wavelength of this spectrophotometer is calibrated by the three polynomial fitting method, and the relative error of wavelength calibration is less than 0.1 nm. This spectrophotometer is used to set up an experimental measurement system, and the chromogenic agent and the standard chromium solution are formulated to test the performances of this system. The results show that this spectrophotometer can meet the application requirements of the online monitoring of hexavalent chromium water quality, which provides a solution for realizing high-precision, multi-wavelength water quality parameter online monitoring.