Abstract: Ultrasonic imaging detecting technique plays a very important role in the field of nondestructive testing in modern industry and it has become an important method for quantitative detecting. Nowadays ultrasound imaging technique is developed to enhance resolution ability. The sound imaging resolution ability has become one of the important performance indexes in evaluating ultrasound imaging system. But it is contradictive between the detectable depth and resolution when taking use of ultrasonic to detect object. In other words, it will get lower detective depth with high frequency ultrasonic system and it will also get lower resolution with low frequency ultrasonic system. Through the discussion in SAR imaging principle, imaging algorithm and orientation distinguishing etc., a theoretic result that imaging resolution is just related with aperture and no related with pulse frequency was demonstrated. It is also revealed the feasibility of applying the SAR theory in NDT system. Beam for single-element synthetic aperture and multi-element synthetic aperture were simulated through Matlab, whose result becomes basis for the future application.