The improved precision measurement method of high voltage charging low voltage measurement is discussed. Based on the theory of high-voltage charging and low-voltage measurement, through constant-current source test mode, the related fast measurement methods are analysed, and the advantages and disadvantages of different algorithms are compared. The practical application scope of different algorithms is determined in order to find the improved precision measurement method. The results show that the combination of saturated magnetic circuit method and constant current source device method can improve the actual measurement accuracy and expand the application scope of high voltage charging low voltage measurement. Therefore, the saturated magnetic circuit method and the constant current source device method are improved precision measurement method, which improves the measurement precision of high voltage charging low voltage through the DC resistance testing device of power transformer.