As one of the widely used structures in modern civil engineering, the cable system, as the main load-bearing component, is crucial to the safety and service life of the overall structure. Magnetostrictive guided wave non-destructive testing technology has the advantages of large detection length and wide coverage of structural areas in cable damage and corrosion detection, and has gradually become a widely used new non-destructive testing method for cables. Magnetostrictive guided wave non-destructive testing technology is relatively stable in qualitative problems of damage and corrosion. The quantitative relationship between defect size and defect echo is related to many factors. This paper conducts experimental research on the notch detection signal of the basic unit high-strength steel wire of the cable, and obtains the relationship between the defect echo reflection coefficient and the steel wire defect cross-sectional loss rate. It is found that it is very similar to a parabola. This research provides a basis for the quantitative evaluation of damage and corrosion of steel wires and steel wire cables by magnetostrictive guided wave non-destructive testing.