The utility model relates to an improved structure of a flaw engraving machine used for making a magnetic leakage detection sampling tube. The improved structure comprises a telescopic rod of the flow engraving machine, an electrode plate, and a leaf spring, wherein the leaf spring is additionally mounted on the back surface of the telescopic rod of the flow engraving machine close to the mounting part of the electrode plate by using an original mechanism of the flow engraving machine, so that the electrode plate keeps to be in contact with the inner wall of the magnetic leakage detection sampling tube all the time. Compared with the prior art, the improved structure has the beneficial effects of simple structure, convenience in manufacturing and good use effect, and the electrode plate of the flaw engraving machine is kept to be in contact with the inner wall of a steel tube all the time, so as to achieve the set depth of an engraved groove.