In the dual-wavelength interferometer, the traditional equivalent wavelength method only uses the difference information between two single wavelength wrapped phases, and ignores their size as well as positive and negative information. The effective wavelength method based on the least common multiple directly uses the wrapped phase information of multiple single wavelengths, and thus not only it can achieve a larger unambiguous measurement range than the traditional equivalent wavelength method, but also it has no error amplification effect. However, the existing least common multiple effective wavelength phase unwrapping algorithm is mainly based on the look-up table method, which is slow and not suitable for a practical measurement. In this paper, a new algorithm is proposed, which can greatly improve the calculation speed of the least common effective wavelength method, so that it can be applied in dual-wavelength or multi-wavelength interferometers.