The invention relates to a device for performing combined pretreatment on activated sludge to produce short-chain fatty acids. The device comprises a gamma-ray irradiation device, an ultrasonic disruption device and a closed fermentation device, wherein the ultrasonic disruption device comprises an ultrasonic generating treatment device, an ultrasonic detector and an ultrasonic controller; the ultrasonic generating treatment device consists of a tank-shaped container and an ultrasonic generator; the tank-shaped container is arranged in the ultrasonic generator; a feeding hole of the tank-shaped container is communicated with a discharge opening of the gamma-ray irradiation device; the closed fermentation device is a closed fermentation tank; and the feeding hole of the closed fermentation device is communicated with the discharge opening of the tank-shaped container of the ultrasonic generating treatment device. According to the device disclosed by the invention, the activated sludge can be subjected to gamma-ray irradiation and ultrasonic pretreatment, and the degree of dissolving proteins, polysaccharides and other organic matters in the sludge in water can be obviously improved due to the treatment, so that a rich water-soluble organic matrix is provided for subsequent sludge fermentation acid production, the acid production rate of the sludge is improved, and the anaerobic fermentation time of the sludge is shortened.