Evanescent wave in total internal reflection has special characteristics and important applications. It is very necessary to research on the characteristics of the energy flow of evanescent wave. But the existing studies are simple or incomplete. In this paper, based on the theory of electromagnetic field, the mathematical expressions of electric field intensity, magnetic field intensity, energy flux density and average energy flux density(AEFD) of the evanescent wave are derived, as the incident light is linearly polarized. By numerical computation, the links between the modulus and direction of AEFD of evanescent wave to the incident angle and incident light vector direction, are analyzed. The conclusions include that the energy flow of evanescent wave is limited within a thin layer near the interface of media, the modulus of AEFD is influenced by incident angle, and there is an angle between the direction of AEFD and incident plane, as well; only as the incident light vector is parallel with or perpendicular to the incident plane, the direction of AEFD of evanescent wave is in the incident plane. These mathematical expressions of these physical quantities and conclusions are helpful for understanding the evanescent wave and doing the related studies.