When the radius of the cylinder is much larger than the wavelength, an approximate expression for the scattering of a Gaussian beam by a conducting cylinder is obtained by using Watson transformation. The beam fields in shadow region and illuminated region are, respectively, derived from residue theorem and stationary phase method. In order to check the theory, comparisons for scattering pattern and field variation in space were made. Numerical examples of the scattering patterns for

being the beamwidth, the radius of cylinder, and the wavelength) are also illustrated.