Parabola modification method of double helical gears was proposed, tooth contact analysis of modified gears including typical install errors was established based on tooth contact analysis of helical gears. Considering the transmission errors and meshing performance, transmission behavior of double helical gears was optimized from using the fast non-dominated sorting genetic algorithm to obtain the best parabola modification coefficient which could also avoid the local convergence. Examples show that path of contact and transmission errors of optimized gears are preferable distinctly. It has an important significance in improving distribution of load of tooth surface as well as reducing gear meshing impact. In the end, the noise value of original gears and optimization ones are obtained by the loaded test, further verifies the optimal method has certain effect on the noise reduction of gears.