The rigid-flexible coupled dynamics analysis model of crane reducer is built,the gear meshing force and bearing force are obtained by using variable step backward integration method,taking the bearing force as load boundary condition of the vibro-acoustic coupled model,the vibro-acoustic coupled analysis BEM model which include structural mesh,acoustic mesh and field mesh is set up in LMS Virtual Lab,the sound pressure at the surface and field of the reducer is got by using full method. The acoustic optimization model of crane reducer is established by setting the box thickness of reducer as design variable,the mass of the reducer as constraint condition,the field pressure as objective function,the optimal design variable is obtained by using normal boundary intersection optimization algorithm. The results show that the peak amplitude of surface sound pressure occur at center frequency of 500 Hz before and after optimization,the sound pressure of the four field points of the reducer are reduced after optimization.