Considering the design of high contact ratio of double helical gears, based on the loaded tooth contact analysis technology, the bending-torsional-axial coupling dynamic model of modification double helical gears was established with the concentrated parameter method in the consideration of the special axial floating installation characteristics of pinion, the stiffness excitation, error excitation and meshing impact excitation. The dynamic equations were deduced by the newton method and the vibration acceleration of gear transmission system was obtained. A fitness approximation genetic algorithm was introduced to dispose the problem of time waste in calculating fitness. The root mean square of vibration acceleration was selected to be object function to optimize the basic design parameters and parabola modification coefficient by the fitness approximation genetic algorithm. 〖JP〗The effectiveness of the optimization method was confirmed by the given example. In the end, the noise values of original gears and optimization ones obtained by the loaded test further verify that the optimal method has certain effects on the noise reduction of gears.