Dual-wavelength or multi-wavelength digital holography extends the axial measurement range to the order of several microns or more, but the noise enlargement effect greatly affects the measurement accuracy. To address this problem, an unwrapping method is proposed based on synthetic-wavelength phase and searching process. This methodology implements noise suppression using phase unwrapping at the single-wavelength level and averaging. We introduce the analytical approach and provide a simulation analysis to demonstrate the effectiveness of this noise suppression method. The proposed method is used to measure a standard step and the results demonstrated the feasibility of the proposed method in practical usage.