A　polarization-multiplexing　off-axis　dual-wavelength　digital　holography　is　presented.　The　structure　of　a　groove　grating　is　measured　by　using　the　polarization-multiplexing　dual-wavelength　digital　holographic　system　with　a　co-path　optical　configuration.　Two　holograms　of　the　grating　is　recorded　at　the　wavelengths　671nm　and　656nm　by　using　a　pair　of　CCDs,　and　then　their　two　phase　maps　are　reconstructed　by　numerical　simulation　of　diffraction　process　of　the　holograms,　respectively.　The　synthetic　phase　image　is　obtained　by　subtracting　two　phase　images　directly.　Thus,　the　unwrapping　phase　map　of　the　grating　is　achieved　by　using　the　polarization-multiplexing　dual-wavelength　digital　holography　in　the　off-axis　optical　setup.