The invention discloses a calibrating method of a color spectrum in oil on-line detecting device, which includes the following steps: step 1, calibrating through the oil sample off-line value at the very day when a transformer is failure-free: 1), sampling an oil sample from the calibrated transformer at the very day, testing concentration of all components and obtaining the average value, calculating and obtaining the correction factor ki,1, and 2), adopting ki for factor correction of detected components of an oil on-line device of the transformer, and completing the calibration; and step 2, calibrating through the color spectrum off-line assay values of several continuous days when a transformer has failure: 1), taking oil samples from the transformer continuously within three to five days, testing concentration of all components and obtaining the average value, calculating and obtaining the correction factor ki1, and 2), adopting ki for factor correction of the detected components of the oil on-line device of the transformer, and completing the calibration. The method better solves the problem that the color spectrum in oil on-line detecting device has big deviation of the on-line detection result for fault early warning of the transformer and off-line data, and is of little difference in analyzing and judging the running condition of the equipment.