As a precise optical frequency ruler, femtosecond optical frequency comb can be used to generate continuous-wave laser with ultrahigh frequency stability. Absolute distance measurement by multi-wavelength interferometry, making use of multiple wavelengths referenced to comb modes, has advantages of fast measuring speed, real time, large non-ambiguity range and high precision. Based on the theory of multi-wavelength interferometry and high frequency stability of optical comb, the second-order synthetic wavelength method is proposed to extend the non-ambiguity range and keep an enough wavelength gap for the wavelength demultiplexing of interference signals. Combined with characteristics of wavelengths in the optical comb, relationship between non-ambiguity range and wavelengths is analyzed, and selection procedure is elaborated. Simulation is performed with a fractional coincidence demodulation algorithm. The result shows that four wavelengths interferometry can achieve a non-ambiguity range of 35.636 mm with fractional phase accuracy of 0.01, and in the case of five wavelengths that range can be up to several hundred millimeters. The latter range corresponds to a dynamic range of 109 level, relative to measurement resolution.