To　realize　online　multiaxial　fatigue　damage　assessment　for　the　mechanical　components　in　service,　an　online　multiaxial　cycle　counting　method　is　proposed　coupled　with　the　segment　processing　technique　and　Wang-Brow＇s　relative　equivalent　strain　concept.　Meanwhile,　considering　all　the　stress　and　strain　components,　which　contribute　to　the　fatigue　damage　on　the　critical　plane,　a　multiaxial　fatigue　damage　model　without　any　weight　coefficients　is　also　proposed　in　an　equivalent　form　of　shear　strain　energy.　Then,　an　online　fatigue　damage　evaluation　method　for　multiaxial　random　loading　is　developed　by　combining　with　the　proposed　damage　model　and　online　cycle　counting　method.　The　experimental　results　showed　that　the　proposed　online　cycle　counting　method　can　be　successfully　applied　to　the　calculation　of　multiaxial　fatigue　damage　under　random　loading.　Moreover,　the　proposed　online　multiaxial　fatigue　damage　evaluation　method　can　provide　satisfactory　predictions.