The invention discloses a comprehensive diagnosis method of an oil-immersed transformer based on a fault tree, which includes steps: (1) collecting correlated condition characteristic parameter values of gas dissolved in oil, moisture in oil, local discharge, iron core grounding current, top oil temperature and winding optical fiber temperature measurement of the transformer in real time, (2) normalizing by aid of diagnosed fault types of gas dissolved in oil, (3) counting fault diagnose accuracy of all diagnose methods, (4) obtaining state quantity of measured gas dissolved in oil for respective diagnose, (5) obtaining a fault confidence digit group, and (6) integrating to obtain a fault diagnose result of the transformer. The comprehensive diagnosis method combines advantages of various oil chromatographic diagnose methods, overcomes defects of a single diagnose method, adopts gas dissolved in oil as a diagnose core, is supplemented by fault tree diagnose, masters operation health conditions of the transformer macroscopically, can achieve fault positioning of devices, and has important practical meaning for finding potential hidden fault danger of devices and ensuring safe and stable operation of an electrical power system.