Micro spectrophotometers are widely used in water quality online analysis instruments, and thestray light is one of the key parameters to evaluat the performance of spectrophotometers.It can effect the datafrom the measurement results.The source of stray light and its effect on the measurement results of the spectrophotometer are analyzed in this paper. The stray light of two spectrophotometers is detected, and the two spectrophotometers are used in the hexavalent chromium water quality analyzer to measure different concentrations of standard solutions. The results show that the larger measurement errorsare introduced due to the stray light, and as the concentration of the solution increased, its impact would become more serious. The suppression and control of stray light should be emphasized in order to improving the measurement accuracy, ensuring the safety of agricultural water, and reducing the various risks caused by water pollution.