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.