The effective method to reduce the soild noise in a cab is to control the vibration of these walls directly. The influence of the wall vibration on noise is related not only to the magnitude of the vibration, but also to its phase and location. After finding the areas which have main influence on noise, it is a good way to reduce the noise effectively by controlling their vibrations. Based on the equation of acoustic pressure in a cab and boundary conditions deduced from Helmholz's equation and Green theorem, the influence of the cab wall vibration on acoustic pressure near driver's ear was analyzed with boundary element numerical analysis method. Then some parts of the walls which mainly have positive influence on reducing ear noise were damped, which result in noise reduction of 2.2 dB(A).