The structural finite element model and soundstructure coupling boundary element model of a transmission box are established in this paper. The modal analysis of the structural finite element model is done. The soundstructure coupling boundary element model is solved by boundary element method (BEM). The distribution of noise in a particular frequency domain is analyzed. Based on modal acoustic contribution analysis (MACA), the identification and effective control of the main noise source of the transmission box are carried out. This work has a practical significance for research and development of this product.