Using the self consistant field (SCF) method, the channel light waveguide problem was solved and the propagation constants with 10 significant figures were obtained. It proves that a simple SCF method for this problem is superior very much to the finite element method (FEM) and the others, both in acurracy and in caculating time, and that it should be used in the advanced light waveguide design. Moreover, the causes why the SCF method can reach high accuracy and the FEM can′t be improved ultimately are discussed in detail.