Based on the 1,5-diphenylcarbazide spectrophotometry, we researched on the influence of pH value on the determination of hexavalent chromium. By adding different amounts of acid into two water samples with different volumes and concentrations, we measured the absorbances of the solutions so as to determine the hexavalent chromium. Through analysis by EXCEL on the measured data, we found that the absorbance reduced with the decrease of pH value. In the presence of high acidity, which means small pH value, the measured hexavalent chromium was smaller than the original value before adding acid. Therefore, the variation of pH value had an impact on the measurement of hexavalent chromium. Moreover, we employed the gray system model GM(1,1) to process the experimental data, and derived a linear equation for the absorbance under different acidities. Through t  test and  F  test, no significant difference was found between calculated absorbance and measured absorbance. This research is helpful to quantitatively looking into the influence of pH value on hexavalent chromium determination.