Railway　axles　are　very　common　structures　and　the　most　important　components　used　in　high-speed　transport　fields,　such　as　high-speed　railway　and　subway.　However,　as　parts　of　vehicles　run　during　the　serving　time,　the　biggest　failure　mostly　comes　from　axles.　For　this　reason,　it　is　of　vital　importance　to　detect　the　original　cracks　existing　in　railway　axles　and　to　recognize　the　damage　severity　or　activeness　of　railway　axles　because　the　initial　cracks　are　usually　not　easy　to　detect　and　know.　Acoustic　emission　(AE)　method　is　one　of　the　most　widely　used　ultrasonic　techniques　for　monitoring　crack　growth　and　structure　activities.　In　this　paper,　AE　technique　is　used　to　analyze　and　evaluate　different　damage　growing　stages　of　40CrNiMoA　steel　in　process　of　stretching.　Parameter-based　approach　was　used　to　describe　AE　signal　and　the　process　of　stretching.　Then,　the　regulation　between　AE　signal　and　crack　growth　was　obtained　by　comparing　AE　parameters　and　stress-strain　diagram　of　40CrNiMoA　steel.