Rhomb-type retarder is a high-precision achromatic λ/4 phase retarder. Because of material dispersion of the refractive index, there is still 2° deviation of the delay in the visible range. In order to meet the demand of precise measurement and applications, the principle of phase retardation with oblique incident angle is expounded, based on the total-reflection phase transformation theory of the phase retardation. By means of Fresnel rhomb, the variation law of the phase retardation for rhomb-type retarder with the incident angle is analyzed. The result indicates that incident angle has obvious influence over the phase retardation.The phase retardation is sensitive to the incident angle, and also has close correlation to incident position of the incident ray, which is asymmetrical to the incident position of the incident ray. So when the wavelength of the incident light changes, only the position of rhomb-type retarder need to be changed to allow the light to represent oblique incidence in the front of rhomb, and then selecting suitable incident angle can compensate the dispersion phase deviation, and make the same rhomb possess standard λ/4 phase retarder under all different wavelengths. Deviation of the delay is less than ±0.009° when the precision of angle adjustment is ±0.01°. The precision cannot be obtained for quartz wave plate or mica wave plate.