This　research　develops　a　robust　experimental　procedure　to　monitor　the　evolution　of　early　fatigue　damage　in　AZ31　magnesium　alloy　with　the　acoustic　nonlinearity　parameter　beta,　and　demonstrats　its　reliability　by　measuring　the　linear　relationship　between　amplitudes　of　the　second-harmonic　waves　and　fundamental　waves　squared.　Using　this　system,　beta　of　two　sets　of　specimens　with　different　stress　level　is　measured.　The　experimental　results　show　that　there　is　a　significant　increase　in　beta　linked　to　fatigue　degree　in　the　early　stages　of　fatigue　life　and　reaches　the　maximum　about　55%　of　fatigue　life,　when　the　stress　level　is　+/-　60%　of　the　yield　stress,　beta　can　characterize　the　early　fatigue　damage　of　magnesium　alloy.　However,　when　the　stress　level　is　+/-　70%　of　the　yield　stress,　there　is　a　regular　fluctuation　in　beta　linked　to　fatigue　degree,　this　experimental　results　can＇t　be　explained.