The invention discloses an ultrasonic flaw detection device for internal defects of a pipeline. The ultrasonic flaw detection device mainly comprises an ultrasonic detection device, a control circuit board and a computer, wherein the control circuit board and the computer coact to regulate and control the operation of the ultrasonic detection device; the ultrasonic detection device comprises a Z-axis stepping motor and a W-axis stepping motor concentric with the Z-axis stepping motor in the vertical direction; an ejection plate is arranged above the Z-axis stepping motor, and connected with the Z-axis stepping motor through a screw-nut transmission mechanism; the bottom end of the Z-axis stepping motor is connected with the bottom mend of the W-axis stepping motor; the other end of the W-axis stepping motor is provided with a probe clamping device; a plurality of ultrasonic probes are arranged at the end part of the probe clamping device. According to the ultrasonic flaw detection device, the ultrasonic probes are used to carry out ultrasonic flaw detection in a steel pipe to be detected, the steel pipe to be detected is fixed, an extra signal coupling device is not required, and the integrated device is simple, small in size, and low in input cost, and has a good application prospect.