Abstract: Toward the problem of the metal material early degradation of mechanical properties,the project study measurement method of nonlinearity parameters,develop a robust experimental procedure.There is linear relationship between amplitudes of the second-harmonic waves and the squared fundamental waves as a function of increasing input voltage amplitude,the experimental system is reliable.Ultrasonic nonlinearity parameters of magnesium samples with some fabricated degradation are measured.After the fatigue life of 30% and into plastic deformation stage of metal material,ultrasonic nonlinearity parameters is sensitive to the degradation of mechanical properties.β can characterize the early degradation of mechanical properties of magnesium,nonlinear ultrasonic measurement has potentially engineering value for nondestructive testing and life prediction of mental materials.