An improved off-axis dual-wavelength digital holography method is presented for three-dimensional surface topography measurement of objects with significant variations. The object beams informations of the two different laser wavelengths are recorded on one hologram based on polarization splitting principle, thus the measurement range can be expanded to micron or millimeter scales by means of the equivalent wavelength induced by difference frequency effect between the two wavelengths and the equivalent wavelength is longer than any single one. So the problems of inevitable phase unwrapping when the object morphology with significant variations are measured by a single wavelength is overcame effectively. The morphology of a staircase sample with micron height variations is measured. The experimental results are consistent with the nominal values and the values measured by profile-system, which shows the effectiveness of the proposed method.