For analyzing the energy loss of an extraordinary beam at crystal surfaces, a simple method is proposed to calculate the corresponding reflectivity and transmissivity. According to the boundary relations of the electromagnetic field, and based on the phenomena of double refraction and double reflection in crystals, the reflectivity and transmissivity are deduced at boundary when an extraordinary beam is incident from an isotropic medium upon a crystal or from a crystal to an isotropic medium with optical axis in the incident plane. The change of the direction of the optic axis has influence on the critical angle θc but little influence on reflectivity, The reflectivity for e-beam incident upon a crystal from an isotropic medium is experimentally verified with LiNbO3 crystal. The formulate given in this paper are general and representative.