The method for determining damage. conversion of main shaft of aeroengine is introduced. In accordance with the linear accumulating fatigue damage theory and S-N curve,the standard damage cycles per flight hours,or damage conversion ratio Ap for flight can he obtained from the comparative residual life tests of the shafts with different service life. The main shaft fatigue life tests indicate that even the worst main shaft can resist the standard load cycle, or predicted safe standard cycle life. If it is divided by Ap,the service life (hours) of a main shaft can be determined. Further, the determination method of target service life is provided for general maneuver flight load conbined with spin flight load.