Abstract: As the most basic connection method in mechanical structures, bolt connection is widely used in large key equipment such as condenser units. Traditional torque method and angle method have defects such as low precision and inability to realize real-time monitoring, while offline detection leads to low efficiency. To this end, this paper develops a portable ultrasonic testing device for bolt preload. The system consists of an industrial tablet computer, an acquisition host and an ultrasonic probe, with performances including measurement frequency>2 MHz, preload measurement accuracy≤±5%, and temperature compensation accuracy≤±0.2 ℃. Experimental verification shows that the device can meet the testing needs of bolts of different sizes and working conditions, providing key technical support for the safe operation and maintenance of equipment.