The simulation and optimization method for an internal combustion engine was proposed, which was used to evaluate the interior noise change before and after the engine noise optimization. The finite element models of the engine and dash panel were constructed and validated by modal analysis technology. The flexible multi-body dynamic method was employed to calculate the engine vibration response with which the structural radiated noise was obtained via boundary element codes. Then, optimal design of the block was implemented to improve its stiffness and separate global modal frequencies and excitation peak frequencies, which aimed to decline the structural response and radiated noise. The acoustic coupled model of the engine and dash panel was constructed to calculate the acoustic power transmitted from the engine surface through the dash panel before and after the block optimization. The results show that the optimal scheme for the block substantially lower the structural radiated noise, as well as that for the engine. The acoustic transfer path between the engine and interior noise finds that the interior noise is distinctly reduced, which proves the feasibility and effectiveness of the optimal design.