The present disclosure relates to a method and system for detecting a structural defect in additive manufacturing. The method includes: layering a three-dimensional model of an additive manufacturing test piece to obtain a two-dimensional contour of an interface of each layer, and generating a machining path; arranging a non-contact sensor at a fixed measuring point of the additive manufacturing test piece, and acquiring an ultrasonic signal at each machining point when a pulse laser conducts machining point by point along the machining path; forming a visual ultrasonic field based on all the ultrasonic signals, and determining ultrasonic field data; determining, based on the ultrasonic field data, a curve of a peak of an incident wave changing with the machining path; and determining whether a machining defect exists at the machining points based on the curve of the peak of the incident wave changing with the machining path.