It is necessary to calibrate the optical path length before using wavelength-tuned phase-shifting interferometer based on fixed step phase-shifting algorithm to compute the phase distribution, as the precision of measurements depends on the step value. To implement the calibration for the interferometer, a new algorithm named two-frame differential average phase-shifting algorithm (TDA) is proposed to calculate phase shift directly by using the interference figure. Simulation and theory analysis are adopted to verify the accuracy and practicability of the algorithm. Furthermore, related experiments are made. The result indicates that by handling the fixed step phase-shifting interferograms with TDA, the result which matches well with the real value can be obtained; the root mean square (RMS) repeatability is better than 0.07 nm(1.106λ/10000), which meets the design index; by comparing the measurement results for the same component with this interferometer and Zygo interferometer, the RMS of the difference is 0.742 nm.