Based on Debye series of an infinite homogeneous cylinder and a multi-layered sphere, and incorporating with the Gaussian beam shape coefficient in the circular cylindrical coordinates, Debye series of electromagnetic scattering by an infinite multi-layered cylinder in a Gaussian beam is discussed, and the formula is obtained. Some numerical calculations are also given. Comparing with generalized Lorenz Mie theory, the results of ours are very well. In general, the Debye series of multi-layered cylinder will play an important role in the study of scattering characteristic and the analysis about sizes and refractive index of inhomogeneous particles.