Abstract: On the basis of the quantitative analysis of the amount of the defects detected when weld ultrasonic test, discussion was made on the change of the equivalent defect diameter under different test sensitivity, and on the equivalent substitute method of different size ultrasonic blocks. It can help test persons to confirm suitable test sensitivity, choose suitable equivalent ultrasonic block and understand the condition of the defects more directly. It can also help test persons to constitute scientific and reasonable test acceptance standards.