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The origin of gamma-ray burst (GRB) X-ray plateau, especially the internal plateau, is still unclear, but it could be related to GRB's central engine of magnetar. It is generally believed that the spin-down power of the magnetar is injected into forward external shock, however we propose here that most of the power will be dissipated behind the GRB jet through larger amplitude electromagnetic wave (LAEMW). Based on this proposal, the relevant physical conditions and observational implications are analyzed and discussed, and various kinds of X-ray light curves could be reproduced. Although it is still a matter of debate about the chromatic multi-band afterglow in the standard external afterglow fireball model, we can explain naturally this feature under this proposal, i.e., the electrons generating the X-ray plateau and emitting the optical afterglow are accelerated by different mechanisms. %Furthermore, we predict that the X-ray emission of spin-down wind could possibly precede the prompt emission of GRB jet if the energy of LAEMW is dissipated first but shock-induced radiation in the jet is produced later. It is emphasized that both the GRB jet and the spin-down wind should have significant observational consequences in the magnetar scenario, and should be focused equally in GRB physics.