Abstract: Permanganate index is one of commonly used to determine the eligibility of water quality testing projects。The experimental studied on the the effect of different dilution factor in the same Water quality-Determination of permanganate index, And has made the error analysis and the result analysis. The experimental results show that I_Mn value is closer to the true value with decreasing dilution factor and the increase in volume of the sample water. Therefore ,the water volume should be increased as much as possible in the process of test so that to ensure the accuracy and reliability of results.