The achromatic wave-plates are widely used in communication, biology, geological, aviation and ocean because their retardation are not vary with wavelength unlike the common ones. This article gives the designed theory for the identical material three-in-one composite achromatic λ/4 wave-plate according to composite wave-plate theory, generalizes the theoretical curve between retardation and wavelength, then a optimum design scheme is put forward based on flat-plate three-in-one composite achromatic wave-plate in response to theoretical curve of retardation. Theory analysis shows that this design can improve achromatic range from 130 to 140 nm which is longer than common ones under the same delay deviation, and advances its achromatic accuracy by adjusting composite angle exactly for specific operating requirements.