Ultrasonic phased array technology becomes significant in structural health monitoring with its flexible beam deflection and strong focusing performance. The transducer is used to complete the multiple directions scanning of the structure. During the process of beam scanning, the signal exciting from each element needs to add a time delay in time domain, due to the difference to the distance from each element to the far field target point, so that the excitation signal can reach the target point and the far field at the same time. Besides, in the process of receiving and transmitting process, it also needs to add a time delay for the response signal, so that the sensor can receive the reflection signal from the preset deflection angle. Therefore, the error will be generated in the calculation of time delay. Making the anti-rust aluminum plate as the experimental object, the process of ultrasonic phased array structural health monitoring will be introduced. And then the reason of time delay error will be pointed out and analyzed.