Abstract: A test-plate with diverse diameters, depths, slopes holes were designed for simulating common defects which were cracks, air holes in real welds in order to demarcate the testing sensitivity for instrument. Compared with results of experiment on test-plate, we found the regularity of sensitivity for detecting diverse defects by ultrasonic phased array. Besides, the testing effect would be improved by adjusting the focal distance properly. This conclusion had a certain reference value in inner defects testing in real welds and steel products.