The invention relates to a portable power transformer fault diagnosis instrument in the power engineering technical field. The fault diagnosis instrument comprises an infrared source, a modulating and light splitting module, an oil-gas separation chamber, a photoacoustic cavity, a phase-locking amplifier, an A/D converter, a laser power meter, a digital recorder and a vacuum pump. An oil sample is subject to ultrasonic degassing separation in the oil-gas separation chamber, fault gas is injected into the photoacoustic cavity, the gas periodically vibrates under the action of an infrared beam which is modulated by mechanical chopped wave and split by a light filter to excite acoustic wave signals. The acoustic wave signals can be amplified by the phase-locking amplifier after being detected by a rheomicrophone, converted into digital signals by the A/D converter and input into the digital recorder, the acoustic wave signals are proportional to the specific gravity of various gas molecules, and display values on the digital recorder are employed to judge whether the current running state of a power transformer is good. The fault diagnosis instrument has the advantages of small volume, light weight, convenient carrying and short detection period.