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A novel simultaneous phase-shifting multi-wavelength interferometry based on the least-square iterative algorithm is presented. From the in-line phase-shifting interferometry system of three illumination wavelengths, a phase-shifting procedure of all illumination wavelengths can be carried out simultaneously. A sequence of phasesshifting interferograms with unknown phase-shifts at multiple wavelengths have been recorded simultaneously by a monochrome CCD. By using the least-square iterative operation, the wrapped phase at each single wavelength can be retrieved respectively, so the phase of synthetic wavelength can be obtained by multi-wavelength optical phase unwrapping and noise reduction, and then the three-dimensional topography of the measured object is got. The proposed method has been validated by both the numerical simulation and optical experiment, which shows simple optical setup, high measuring accuracy and capacity of anti-noise.