A method uniting the scalar approximation with the Snell's law and the Fresnel relations is presented in detail. Two formulae of the energy flux density and the diffraction efficiency are derived for the step transmission grating. As a example of application, the diffraction efficiency and the factors influencing on them are analyzed and calculated for the transmission grating with 4 steps. The results show that it is excellent for the gratings of d>4λ, because this kind of the method can not only avoid the complexity of the complete arrow wave theory, but also overcome the insufficiency of the pure scalar approximation. It isn't suitable for the gratings of d<4λ.