To reduce the vibration and noise of double helical gears transmissions,its dynamic behavior was studied.Considered the internal dynamic excitation produced by stiffness excitation,error excitation and mesh impact excitation,a bending-torsional-axial coupling dynamic model of double helical gears was established by the method of concentrated parameter.The dynamic equation of this system was deduced by the Newton method.The effect of excitations and tooth modification on dynamics behavior was discussed,and the corresponding conclusions were obtained.Finally,a test was carried out and the result agrees well with the theory analysis,which validates the correctness of the dynamic model.