It is introduced that when the COD_Cr concentrations both before and after the treatment are determined by using potassium dichromate method in the wastewater treatment plant laboratory, the concentration of potassium dichromate solution is adjusted to 0.050 0 mol/L and the concentration of ammonium irons(Ⅱ) sulfate hexahydrate solution to 0.02 mol/L. This will not influence the determination result, but can reduce the essay cost, save working hours and reduce the secondary environmental pollution caused by the wastewater from the laboratory.