A transformer heat exchanger adopts the evaporative cooling technology, wherein transformer oil (80) enters into an oil chamber (85) through an oil inlet flange (110), and then flows out from an oil outlet flange (70) after exchanging heat with evaporating liquid (90) through a metal heat exchange tube (20), the evaporating liquid (90) is filled into a space which is formed by a liquid-collecting space (50) and the metal heat exchange tube (20), then is continuously heated up by heat transformer oil (80), and can absorb heat energy after boiling and vaporizing to generate phase change, thereby realizing the purposes of heat absorbing and cooling. Evaporating liquid steam which is generated through vaporizing upwards enters into a gas-collecting space (120) through the metal heat exchange tube (20), flows out the heat exchanger through a gas outlet flange (130), regenerates the phase change into liquid through the secondary cooling, and then returns to the liquid-collecting space (50) through a pipe, thereby finishing the circulation. The invention can increase the heat exchange efficiency, can not generate the transformer insulation accidents, even if the evaporating liquid leaks into the transformer oil owing to the higher electric insulation level of the evaporating liquid, thereby avoiding safety hidden dangers caused by the accident.