Additive manufacturing (AM) is a rapidly developing technique which presents both substantial opportunities and challenges for manufacturing industry. However, the internal defects will influence the mechanical behavior of AM structures, and thus characterization of the defects of AM materials becomes an interesting issue. In this study, a non-destructive testing technique for AM structures is proposed based on the reflective coherent gradient sensing (CGS) method. Using this method, the slope of a reflective surface can be measured accurately. In the study, the measured structure is firstly prepared a reflective film using replication technique, and then is exerted an external load. The full filed slope of the structure can be measured using CGS and will show a turbulence which can be used to determine the location of the defects of structure (pores or cracks). Using this technique, the spatial resolution of defect size can reach 200μm. From the experimental results, the method shows the merits of full-field, non-contact and real-time measurement. The successful experimental results show that reflective CGS method is effective for detecting the internal defects of AM structures, and will have a good prospect of further application.