Achromatic retarder is a device to be used in wide spectral range for optical phase retardation. The phase retardation is constant when the wavelength is changed. Modern polarization technology and optical modulation demand high precision for the measurement of achromatic retarder. Based on the principle of polarizing modulation, this paper sets up an experiment system for measuring the phase retardation for high precision achromatic retarder of rhombic shape. The results indicate that the principle of high precision achromatic retarder is correct and the measurement error is less than 1%, it reaches a high precision and satisfies the requirement of application.