Abstract: In order to improve the detection rate of the grouting density of the pre-stressed concrete beam and ensure the safety of the operation, the ultrasonic wave method techniques for hidden hazards is studied. According to the actual situation of the pre-stressed concrete beam of the subway, different stirrup and reinforcement ratio are designed in the test beam respectively, and artificial defects of different material types and sizes are preset in the test piece. The results show that the ultrasonic method has good effect on the flaw detection of the subway pre-stressed beams, and the minimum defect within the size of 5 cm can be detected. The field test results of pre-stressed beam in one line in Beijing are in agreement with the simulation test.