The invention belongs to the field of fuel rod control and particularly discloses a method for improving stability of an ultrasonic flaw detection device for fuel rod cladding tubes. The method includes steps of perforating out sides of the fuel rod cladding tubes, arranging high-strength APP integrated tube connectors, putting in a constant-temperature water tank, detecting longitudinal flaws on pipe walls and the like. The method has advantages that stability of the ultrasonic flaw detection device is conveniently and effectively improved, and accuracy in flaw detection is improved.