The invention relates to an achromatic phase delayer which is characterized in that the delayer comprises more than two cascaded sub-wavelength gratings. The functional relation between the actual phase delay alpha lambada (di,hi, theta i, fi) of the wavelengths in the mid-infrared and far-infrared dual bands as for the cascaded sub-wavelength gratings and the target phase delay alpha is sigma lambada |alpha lambada (di,hi, theta i, fi)- alpha|=Min, wherein the di,hi, theta i, fi are respectively the cycle length, the etching depth, the fast-axis direction angle and the duty ratio of the ith sub-wavelength grating, and the Min is the minimum value of the difference between the actual phase delay of the wavelengths in the mid-infrared and far-infrared dual bands as for the cascaded sub-wavelength gratings and the target phase delay alpha. The achromatic phase delayer can acquire the polarization information of the target in mid-infrared and far-infrared dual bands and is suitable for an infrared dual band polarization imager.