Diaphragm gas meter is an instrument which is widely used for measuring gas volume. Valve bonnet and valve seat are the main factors of its metering error. Whereas surface roughness determines the performance of valve gas tightness. Non-contact methods of surface roughness measurement were studied. It was found that spectral confocal method was more suitable to measure the surface roughness of valve bonnet. Then, the surface roughness of valve bonnet which was on a two-dimensional air floating stage was measured by spectral confocal method. Comparing the measure results with the data gotten by stylus profilometer produced by ACCRETECH, there is little error between them. The results demonstrate the feasibility of spectral confocal method to measure the surface roughness of valve bonnet, which provides an effective way to detect the gas tightness of valve bonnet working face.