Based　on　strain　parameters,　a　weight　function　method　is　proposed　to　determine　the　critical　plane　under　variable　amplitude　axial-torsion　loading.　The　critical　plane　is　determined　by　averaging　the　maximum　absolute　shear　strain　plane　during　the　time　for　one　block　loading.　The　averaging　process　is　conducted　through　a　weight　function,　which　is　proposed　based　on　the　maximum　shear　strain.　Then,　combined　with　the　life　prediction　model　and　Wang-Brown＇s　cycle　counting　method,　the　proposed　method　is　used　to　predict　the　fatigue　life　under　variable　amplitude　loading.　The　experimental　data　of　En15R　steel　and　7050-T7451　aluminum　alloy　are　used　to　verify　the　proposed　method,　and　a　good　result　is　obtained.