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We present a new method to remove the typical linear energy-chirp of an ultra-short relativistic electron beam generated from the plasma based accelerator by sending it through short homogeneous tenuous plasma.The strong self wake excited in the plasma can be a linear decelerating field when the length of the beam is much shorter than the plasma wavelength,which can "dechirp" the electron bunch.We obtain the expressions for the final minimum root-mean-square energy spread of the electron beam with different bunch profiles using theoretical analysis,which are in good agreement with the three-dimensional particle-in-cell simulation results.Application of such a simple and flexible method can reduce the relative root-mean-square energy spread of a~10kA,~10fs electron bunch from 1%to 0.1%,resulting in significant improvement in beam quality and offering the potential to drive the future compact x-ray free electron lasers or Tev-class colliders.