This study uses the temperature model algorithm and the equivalent aging factor algorithm to find the correlation between the load and temperature. The program is programmed in a single-chip microcomputer and combined with circuits to measure and analysis the operation of transformer. The detector is used to measure the load state and ambient temperature of the oil-immersed transformer, and then calculate the equivalent aging factor and residual life. Finally, the measured value and calculation result will show on the display. The actual value of the residual life is compared with the theoretical value, which proves the applicability of the measurement of the measuring instrument and the correctness of the calculated value.