Abstract: According to the structure characteristics of inserted fillet welds, a new testing method is presented in this paper which puts the flexible phased array in the inner side of tube. The flexible phased array water-steel model is established. The simulation of the acoustic field and defect response by CIVA software is completed to determine the phased array parameters. At last, based on simulation result, the probe is manufactured. Also, the sample of inserted fillet welds is manufactured. The testing experiments on the sample are accomplished. The results indicated that this technique could effectively detect the crack, slag, porosity defect etc. The detection efficiency is greatly improved and the engineering application is expected to be realized.