According to the testing methods of inserted fillet welds which usually used in the large petrochemical equipment. The flexible phased array probe placed on inner wall of the tube is put forward in this paper. To control the probe to complete the 360 degree circumferential scanning and axial moving, one ultrasonic mechanical scanning device is developed. This device can accomplish the whole two dimensional section C scan imaging.At last, verified the practicability of the device by successfully detected the longitudinal crack flaw in test experiment.