In traditional dual-wavelength digital holographic microscopy (DHM), a synthetic wavelength is obtained by using two lasers with different wavelengths, and the measurement range of the samples’ step height can be expanded from nanometers to micrometers. However, the measurement accuracy reduces along with the expansion of the measuring range, and significant noise is simultaneously introduced in this process. For cases where the sample’s step height is smaller than the wavelength of the illumination light, the measurement accuracy is very important. We present a new approach for dual-wavelength DHM. The synthetic wavelength is shorter than either of the two lasers, and thus higher measurement accuracy can be achieved. The numerical simulation and experiment results show the feasibility of this technique.