The purpose is to discuss the influence of the value of the power supply voltage on the measurement of the internal resistance of the modified meter when the replacement method is used to measure the internal resistance of the meter in the experiment of the meter modification． Through theoretical calculation and experimental research，we found that the essential reason for the increase in measurement error caused by large power supply voltage is that the protection resistance accounts for too high a proportion of the resistance of the whole circuit．The research results of this paper provide an improvement direction for improving the accuracy of resistance measurement by substitution method．