An achromatic phase retarder can obtain the desired phase retardation in a wide spectral range, and it is one of the core elements used in polarization light modulation. A design method for broadband achromatic phase retarder is proposed based on sandwich-type subwavelength metal dielectric gratings according to the rigorous coupled wave theory and the genetic algorithm. A phase retarder which can achieve achromatism is designed in the wavelength range of 900~1200 nm. The maximum deviation of phase retardation around 90° is less than 2.3%, and the diffraction efficiencies of transverse electric wave and transverse magnetic wave are both higher than 90%. The numerical analysis shows that the designed achromatic phase retarder has a large manufacture tolerance for groove depth and incidence angle. The achromatic phase retarder is simple in design, stable in performance and has an important application value.