Metal additive manufacturing,as a nearnet forming process that directly prints raw materials such as metals into products,has extremely high application value in aerospace,automotive and shipbuilding,micro-nano manufacturing,biomedical engineering and other fields. Its special layer-by-layer printing mechanism makes the manufacturing process defect prone to be large,which may cause many problems such as density difference,anisotropy,local deformation and stress concentration.Therefore,reasonable and efficient non-destructive testing methods are needed for defect detection and quality inspection. In this paper,three methods of ultrasonic,radiography and visual inspection are taken as the main objects. By analyzing their principles,typical application scenarios,advantages and limit-ations,the applicable scenarios and development trends of non-destructive testing in metal additive manufacturing are summarized,and the development direction of intelligent detection and control in metal additive manufacturing is proposed.