As the relationship between output intensity and modulation voltage of a modulation laser in Fizeau interferometer will effect the accuracy of phase calculation, this paper proposes an intensity self-calibration phase-shifting algorithm. Firstly, the relationship between output intensity and modulation voltage of the modulation laser was analyzed, and a mathematical model was established. Then, on the basis of least square estimation, the intensity self-calibration phase-shifting algorithm was deduced. Finally, a simulation experiment was performed. The 12 interferograms with changed backgrounds were generated by computers, and their phases were retrieved. The experiment indicates that the algorithm immunes the intensity change well, and retrieves the phase accurately. The measuring results for a flat mirror with a caliber of 100 mm show that the calculated phase precision is RMS of 0.005λ, and PV of 0.073λ. As a comparison with a ZYGO interferometer for the same flat mirror, the deviations are RMS of 0.0014λ, and PV of 0.022λ. Both simulation and experiment results show that the proposed intensity self-calibration phase-shifting algorithm is feasible and practical in the Fizeau interferometer with wavelength tunning.