Achromatic retarder is an important optical device. Because of its important applied value, modern polarization technology and optical modulation demand high precision for the design and measurement of achromatic retarder. Based on the principle of polarizing modulation, this paper presented an autocontrol experiment system to measure the phase retardation for high precision achromatic retarder, whose shape is rhomb. The experimental results indicate that the principle of high precision achromatic retarder is correct and the measurement error is less than 1%, Since the autocontrol experiment system is convenient to operation, it can satisfy the demand of application.