Taking a non-ideal waveguide as an ideal waveguide in the presence of polarization currents giving rise to electric and magnetic fields, we can directly solve the problems of mode coupling and radiation loss of an optical waveguide. By using this method, we have calculated the radiation loss of single mode fibers caused by periodical and random non-uniformities of the core index. It is an alternative method of coupled mode theory, but is mathematically impler and physically more intuitive than the coupled mode theory. At last, taking bending and microbending loss for single mode fibers as an example，wo see that this method also applied to problems which can be solved by generalized coupled mode theory.