The operating modal of a gearbox is investigated experimentally based on the LMS system.With the least square complex frequency（LSCF） method

the parametric identification of each gearbox modal is deduced.Subsequently

the additional input peaks are identified with the analyses of operating modal

and the fake modals caused by harmonic are removed.Through the analysis and discussion of experimental results

assembly eccentricity lead to the spread of gear meshing frequency and harmonic.Finally

according to the characteristics of each modal shape

the running states of gearbox are evaluated

and improved method for the design of gearbox structures is proposed.