Abstract: As one of the landmark technologies of the new round of industrial revolution, additive manufacturing is listed as a new growth point of future industrial development by all countries in the world. However, it is difficult to control the surface quality and easy to produce defects in the process of additive manufacturing. In this paper, the laser ultrasonic longitudinal wave detection method was used to detect the additive defect test block, and the variational mode decomposition method was used to process the defect signal. The results showed that the proposed method can significantly improve the signal-to-noise ratio of the defect signal, effectively detect the additive parts, and lay a solid foundation for the online detection of additive parts.