The invention relates to a method for detecting axial rotating defect of moment instrument of an optical head, firstly outputting sine signals at different frequencies; driving the moment instrument to dither in the focusing or tracking direction; casting a laser beam upon a reflecting point of an objective of the moment instrument to measure the dithering speed at the reflecting point; integrating the speed signal to obtain displacement response signals of the reflecting point by the excitation of the sine signals, drawing amplitude-frequency characteristic curves for focusing and tracking coils of the moment instrument and obtaining first order resonance frequencies of the focusing and tracking coils; according to the amplitude-frequency characteristic curves, judging the axial rotating defect of the moment instrument. The method is used to improve production technology and ensure the quality of product.