The　structural　damage　in　the　main　shaft　threatens　the　service　safety　of　the　wind　turbine　directly.　The　mechanical　properties　of　the　in-service　main　shaft　tend　to　degrade　severely　due　to　the　transverse　cracks　extending　from　the　outer　circumference.　In　the　health　monitoring　of　the　shaft,　it　is　of　great　importance　not　only　to　identify　the　cracks,　but　also　to　accurately　evaluate　the　crack　size.　In　this　research,　a　novel　quantitative　method　is　proposed　for　the　transverse　crack　characterization.　The　diffracted　waves　from　the　crack　tip　received　in　the　central　hole　of　the　shaft　are　used　to　construct　elliptic　trajectories　of　beam　path.　The　crack　position　and　its　depth　can　be　determined　by　the　intersection　of　the　trajectories.　Furthermore,　a　crack　size　quantification　model　is　established,　and　the　numerical　demonstration　is　given　for　evaluation　of　crack　position　and　depth.　Meanwhile,　an　acoustic　finite　element　model　is　developed,　in　which　the　path　of　the　acoustic　waves　radiated　from　the　end　face　of　the　shaft　is　analyzed　in　detail.　The　quantitative　method　is　well　confirmed　by　the　simulation　of　cracks　at　different　locations　with　different　depths.　Considering　the　accuracy　of　crack　evaluation　from　a　large　size　zone,　the　time　of　diffracted　echo　is　calibrated　by　measuring　the　time　shift　of　the　transducers.　A　shaft　sample　with　a　transverse　crack　was　used　to　implement　the　experimental　verification.　Totally,　the　sizing　error　is　less　than　5　mm,　which　indicates　that　this　proposed　method　is　effective　for　the　evaluation　of　surface　cracks　in　the　main　shaft.