Phase extraction precision directly affects the accuracy of phase-shift interferometry. In the wavelength tuning interferometry, a phase calibration is necessary for the test system, because phase shift is determined by interference cavity length and wavelength variation. But an accurate phase calibration is not easy to achieve. The phase shift error can be caused by an inaccurate calibration, which can decrease the phase extraction precision. A phase extraction algorithm based on iteration is proposed. This iteration algorithm does not need to calibrate the phase of the test system, and it can obtain the testing phase and phase shifts through alternate iteration method under the condition that the testing phase and phase shift are unknown. A series of simulations are done to compare the phase extraction accuracy, and the results suggest that the phase extraction accuracy of the proposed algorithm is quite better than that of the traditional four-step phase shift algorithm.