In this paper, a method to determine the optimal rib layout of gearbox for the noise reduction is proposed based on acoustic contribution analysis and topology optimization. Firstly, the radiated noise is simulated using the finite element method (FEM) and boundary element method (BEM). The field point with maximum sound pressure is taken as the objective field point. Secondly, the surface of gearbox is divided into different regions and the region with maximum acoustic contribution to the sound pressure on the objective field point is found by acoustic transfer vector analysis and acoustic contribution analysis. Thirdly, the topology optimization model is established to reduce the velocities on the region with maximum acoustic contribution. Lastly, the topology optimization model is solved using the SIMP method and the ribs can be arranged according to the results of topology optimization. The simulation results show that the sound pressure on objective field point is reduced remarkably by using this method.