The ultrasonic computed tomography(USCT)method is derived from the basic principles of X-ray section scanning.This method records the arriving times of ultrasonic wave between the probes and the sources to calculate the elastic wave velocity values in the section using the arrival times.Through analyzed the distribution of elastic wave velocity in aim area,the information of the strength and the homogeneity of the investigated zone could be got indirectly.The authors introduced the operational principle of USCT and a practical case of using this method to detect the interior defects in large scale concrete structural member.Compared with other exploration methods,this method is more efficient and accurate. The ultrasonic computed tomography (USCT) method is derived from the basic principles of X-ray section scanning. This method records the paging times ultrasonic waves between the probes and the sources to calculate the elastic wave velocity values ​​in the section using the arrival times .Through analyzed the distribution of elastic wave velocity in aim area, the information of the strength and the homogeneity of the investigated zone could be be indirectly. These authors introduced the operational principle of USCT and a practical case of using this method to detect the interior defects in large scale concrete structural member. Compared with other exploration methods, this method is more efficient and accurate.