In this paper we suggest a simple structure consisting of a thin foil and a solid target with specified gap width to achieve the laser-field amplification, and study the trapping process of a single pulse. Both the theoretical and simulation results show that the EM fields are trapped as standing waves between two layers of enhanced electron density. The laser energy can be accumulated like the accumulation of charges in capacitor, a stably trap is reached after several oscillations. The compressing and the expanding of the electron layer, generated by the incident light pressure, the trapped light pressure and the charge electrostatic fields, are the direct reason for the energy oscillations.