The dynamic model of the gear transmission system is built in consideration of the time-varying mesh stiffness and the gear errors. Then the time history of the dynamic load of system is obtained. The gear box structural-acoustic coupling system is built and calculated by using BEM. The noise spectrum of the gear box and the panel contribution are obtained. The effect of the gear box structure improvement, structure damping and different gear style on the noise radiation is analyzed. The study provides useful theoretical guideline to the design of the gear box.