Increase in the concentration of metal copper and iron ions in the insulation oil for running transformer not only leads to the deepening of the corrosion of the high-voltage copper windings and iron cores immersed in the transformer oil and reduction of insulation performance of the transformer oil，but also causes the increase in the insulation aging ability of transformer oil-paper of metal ion catalysis．In order to track the concentration of copper and iron ions in the transformer oil in operation，and reduce sudden transformer insulation failures caused by the accelerated aging of transformer oil-paper insulation，an inductively coupled plasma mass spectrometry（ICP－MS） with reverse phase switchable hydrophilic solvent liquid－liquid microextraction was developed for the simultaneous separation，enrichment and determination of Cu,2+, and Fe,3+, in transformer insulating oil．The switchable hydrophilic solvent triethylamine（TEA） was used as the extractant，and nitric acid solution was used as the trigger. The extraction conditions were optimized as follows：50 µL of TEA was used to extract 25 mL of transformer oil sample，the molar ratio of TEA to HNO,3, was 1∶1.5，the concentration of HNO,3, was 0.60 mol/L，the reaction time of vortex extraction and the hydrophilicity switching reaction were both 5 min，the lower water extract phase could be directly injected for determination after centrifugation was performed at 4 500 r/min for 5 min. There were good linearities for Cu,2+, and Fe,3+, in the range of 0.100－10.0 µg/L，with their correlation coefficients（,r,2,） of 0.999 6．The detection limits for Cu,2+, and Fe,3+ ,were 16.8 ng/L and 29.0 ng/L，respectively，and the enrichment factors were about 24 times．The concentration of Cu,2+, and Fe,3+, in the oil samples of the simulated aging transformer were determined．And the spiked recoveries of Cu,2+, and Fe,3+, ranged from 93.3% to 112%，with relative standard deviations（RSDs） of 3.6%－5.6%．There is no significant difference between the sample pretreatment method established in this paper and the ashing method used in the current standard，and it is helpful for the online tracking determination of trace Cu,2+, and Fe,3+, in the operating transformer insulating oil．