Achromatic retarder is such a device that can be used in wide spectrum in optical phase retardation. Based on the total-reflection phase transformation theory of the phase retardation, the principle of achromatic phase retardation with oblique incident angle is expounded. And based on the detailed design of Fresnel rhomb, the parameters which induced the change of phase retardation are analyzed. The new achromatic retarder with the specific incident angle and high precision is designed by choosing suitable material in this papaer. It is shown at the achromatic property curve within the range of 365 nm to 1150 nm theoretically that the retardation error is smaller than 0.4°, which is nearly one-fifth of the ordinary Fresnel rhomb phase retardation. So the new design retarder can be used in wide spectrum, at the same time it has the characteristic of small retardation error and high precision.