Traditional fluorescence analysis and detection methods of transformer insulated oil quality, which use fluorescence spectrophotometer to collect the full bands’ fluorescence spectrum of the oil sample and establish the diagnostic model of transformer operation states using the full bands’ fluorescence characteristics of insulating oil with a different aging degree, have the problems of large volume and the high price of photometer and the inability to realize real-time monitoring due to a long time of spectrum acquisition, a new method to detect the quality of transformer insulating oil based on the fluorescent double-color ratio to extract fluorescence characteristic dual band information and establish fault diagnosis model of transformer operation was raised to solve it. Thus, the traditional fluorescence spectrophotometer can be replaced by custom filters and visible photodetectors to realize the rapid acquisition and processing of double-color fluorescence information, which can meet the on-line monitoring and reduce the hardware cost. The aging of transformer insulating oil caused by discharge breakdown fault was analyzed by fluorescence analysis. Different discharge breakdown conditions were simulated. NYNAS oil samples with different discharge breakdown times (10, 30, 50, 70, 90 and 120 min) were prepared as fluorescence detection targets. Fluorescence emission spectra at different excitation wavelengths were collected by fluorescence spectrophotometer, and the optimal fixed excitation wavelength was found to be 280 nm. The 3-point moving mean smoothing method was used to smooth the fluorescence spectrum of the samples and by analyzing the variation of the fluorescence characteristic peak of the oil sample under different discharge breakdown times, bands of 380388 and 399407 nm were selected as the double-color information extraction band. The fault diagnosis model of transformer insulated oil discharge based on fluorescent double-color ratio was established by least-square curve fitting. The study results demonstrate the effectiveness of the fluorescent double-color method on the fault diagnosis of transformer insulated electric oil breakdown, which provides a theoretical and practical basis for establishing a small, low-cost, fast and effective online monitoring system.