Abstract: The paper, through theoretical analysis and test comparison that the applicable contrastive reflectors in calculating the cross bore weld defect equivalent aperture in steel structure weld penetration were the followings, such as RB,  3 mm long cross bore in CSKⅠC series test block and  1 mm short cross bore in CSKⅢA test block. It was revealed in our experiment that the surface energy consumption of RB, CSKⅠC series test block, CSKⅢA test block and the check bar for test was 4 dB and the DAC curve was made when the dB values of RB,  3 mm long cross bore in the CSKⅠC series test block with various depth as well as the reversed echo of  1 mm short cross bore in CSKⅢA test block were reduced by 4 dB respectively. By echo sound pressure analysis of Ultrasonic Cross Bore Theory, the formula for calculating the defect equivalent aperture of long and short cross bore in steel structure weld penetration was derived. As to the erect bore defect, the dB curve and regression function were made for making erect throughhole reflector with steel plate of various aperture and thickness. The fault detection was made with primary and secondary wave for medium plate and with tertiary wave for thin plate. The method of Sorting by Insertion, the defect equivalent calculating method, was applied in calculating the equivalent aperture and was tested.