本文由波动方程推导出超声纵波在不同介质界面的入射波与反射波的振幅关系，振幅与两介质的波阻抗和入射角有关.利用COMSOL建立超声纵波探伤模型，对界面处的纵波传播特性进行有限元仿真，并设计实验验证.在不考虑掠入射的情况下，超声纵波由波阻抗高的波密介质入射至波阻抗低的波疏介质，反射波相对于入射波在界面处发生了π的相位突变，出现半波损失现象.反之，界面处反射波与入射波相位一致，无半波损失.本文从理论、仿真和实验三个方面讨论了超声纵波在不同介质界面处的半波损失现象，为超声纵波在无损检测领域的应用提供了重要的理论支持和实验依据，同时也为相关教材及工业应用中的教学案例提供了有益的补充.

 This paper derives the amplitude relationship between the incident and reflected waves of ultrasonic longitudinal waves at the interface of different media based on the wave equation. The amplitude is related to the acoustic impedance of the two media and the incident angle. Using COMSOL, an ultrasonic longitudinal wave detection model is established to perform finite element simulations of the longitudinal wave propagation characteristics at the interface, and experiments are designed for verification. In the case of no grazing incidence, when an ultrasonic longitudinal wave is incident from a high-impedance (dense) medium to a low-impedance (sparse) medium, An abrupt phase shift of π occurs at the interface relative to the incident wave, resulting in a half-wave loss phenomenon. Conversely, when the interface is between two media with similar impedances, the reflected wave and the incident wave are in phase with no half-wave loss. This paper discusses the half-wave loss phenomenon of ultrasonic longitudinal waves at different media interfaces from theoretical, simulation, and experimental perspectives, providing significant theoretical support and experimental evidence for the application of ultrasonic longitudinal waves in non-destructive testing. It also offers a valuable supplement to relevant textbooks and teaching cases in industrial applications.