This paper is devoted to numerical simulation of a charged particle beam submitted to a strong oscillating electric field. For that, we consider a two-scale numerical approach as follows: we first recall the two-scale model which is obtained by using two-scale convergence techniques; then, we numerically solve this limit model by using a backward semi-Lagrangian method and we propose a new mesh of the phase space which allows us to simplify the solution of Poisson's equation. Finally, we present some numerical results which have been obtained by the new method, and we verify its efficiency through long time simulations.