Two individual digital holograms are recorded by using two different wavelengths, respectively, and two phase images for the wavelengths are obtained after numerical reconstruction. Subtraction of the two phase images corresponding to two wavelengths can yield a new phase image for the beat wavelength. The final phase image is free of 2π discontinuities after dual-wavelength phase unwrapping. This method is explored by numerical simulation, and then a dual-wavelength digital holographic experimental setup is constructed. In the experiment, two lasers of different wavelengths of 660 nm and 671 nm are used to make phase contrast imaging to a standard quartz flake and a plano-concave lens. Continuous phase images are obtained by dual-wavelength phase unwrapping method. The experimental results are in good agreement with the numerical simulation. The effectiveness of dual-wavelength phase unwrapping is demonstrated.