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 calculated. The gearbox structural-acoustic coupling system is built and its noise radiation is calculated by using BEM. The noise spectrum of the gearbox and the panel contribution are obtained. The influence of the gearbox 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 gearbox.