Composite materials have been widely used in many fields. Composite structures are becoming more and more complex, their sizes are becoming larger and larger, the quality requirement is becoming higher and higher, thus resulting a higher requirement on accuracy and reliability of non-destructive testing. Through advanced ultrasonic visual detection technique, the internal defects and structural details of composites can be visualized with high precision, which is very conducive to improving the accuracy and reliability of composite detection, and realizing digital evaluation of internal quality of composites. In this paper, an accurate digital evaluation method for defects in composites based on our developed mono-pulse ultrasonic (MU) visualization technique is proposed. The research and application results show that the MU detection and digital defect evaluation methods can significantly improve detectability and detection resolution of composites. It is more helpful to accurately reveal internal defect behavior of composites, achieve accurate visual inspection and digital evaluation of internal quality of composites. Ultrasonic detection dead-zone and lateral resolution reach to a single ply thickness (approximately 0.125 mm). The similarity between the detected and the design value of defects is close to 1.0. It can realize accurate defect detection and digital evaluation of composites. It has been applied in practice.