The invention relates to an online fault photoacoustic and spectroscopic monitoring system for a power transformer in the technical field of power engineering, comprising 7 tunable cascaded diode lasers, a gas and oil separating room, an optical-acoustic chamber, a lock-in amplifier, a power supply controller of the tunable cascaded diode lasers, a difference amplifier, a convex lens, a laser power meter, a digital recorder, an oil feed-back pump, an electric vacuum pump, a PLC and a computer. Ultrasonic degasification separation is performed for an oil sample in the gas and oil separating room, and fault gas is infused in the optical-acoustic chamber, after being directly modulated, a power supply can generate periodic vibration under the action of an infrared beam, thereby exciting to form a sound wave signal. After being detected by a rheomicrophone, the sound wave signal can be magnified by the lock-in amplifier and then is input in the digital recorder, thus the power transformer can be judged whether the current running state is fine according to the numerical value displayed on the digital recorder. Meanwhile, the data can be transmitted to a remote computer in a wire and wireless communication way.