In this paper, a Gaussian distribution for electron energy is deduced by Fokker-Planck approximation to the Boltzmann equation for high-energy electrons penetrating in multi-constituents media, then a recursion-iteration algorithm for the mean energy calculation of high-energy electron beam is obtained after introducing the modified CSDA mean energy and using Yang's multiple scattering theory. Some calculational results of this algorithm are also given in the article, and compared with corresponding data of Monte Carlo simulations and experimental measurements. The comparison shows that the algorithm can precisely predict the mean energy of high-energy electron beam penetrating in light media. Furthermore, two common formulae for electron beam mean energy calculation in radiotherapy dose algorithms. i. e., the Harder formula and Brahme formula, are discussed, and a more accurate semi-empirical formula is recommended in this paper as well.