For the grazing incidence of a narrow beam, a part of plane wave components can not reach the reflecting interface directly, so they can not be reflected and transmitted in the traditional way. The inverse mode of reflection and transmission is proposed in this paper to describe the interaction between the non-incident plane wave components and the reflection region. Based on the two modes, the plane wave expansion method is utilized to derive exact distribution of the output optical field. Theoretical calculation agrees well with the simulation result which employs the BPM algorithm.