White-light spectral interferometer for measuring physical thickness and reflection phase of thin film is designed. A novel algorithm is also presented to compensate the phase retrieval error in traditional signal processing method. Exact reflection phase is retrieved via a multiple variable optimization algorithm,and the result of linear fit is used as the initial guess. Numerical simulation is performed to demonstrate theoretical availability and high precision of the algorithm. A group of single layer TiO2 thin films are measured and processed with this method,and the retrieved physical thickness results fit the results by photometry very well. The system and algorithm presented provide a new way for fast measurement of thin film thickness.