An　optimization　algorithm　is　developed　to　extend　the　unambiguous　range　of　DWDH,　which　is　suitable　for　imaging　living　onion　epidermal　cells,　and　meanwhile　to　reduce　the　noise.　In　the　experiments,　the　holograms　with　two　wavelengths　can　be　acquired　in　a　single　shot　recording　by　using　off-axis　dual-wavelength　digital　holography,　resulting　to　obtain　their　two　sets　of　spatial　spectrum　via　Fourier　transform　for　these　two　holograms.　The　denoised　unwrapping　phase　image　of　sample　is　straightforward　reconstructed　with　the　optimization　algorithm,　instead　of　increasing　the　noise　due　to　longer　synthetic　wavelength.　As　a　result,　the　unwrapped　phase　images　of　onion　epidermal　cells　are　achieved.