Inner defects of object are cracks, impurity, fatigue damage and corrosion under surface. The defects are secluded so that they deteriorate the quality of product and safety of operation seriously. Taking optical components and aircraft blades as examples, we analyze the forming mechanism and the characteristic of inner defects. Non-destructive detection of inner defects is categorized into energizing and unenergizing non-destructive detection. This paper focuses introducing the principles and research achievements of thermal excitation non-destructive detection and magnetic pumping non-destructive detection.