To improve the phase reconstruction accuracy of two-step phase-shifting interferometry measurement method, a phase blind demodulation algorithm based on zoned background estimation is proposed. In the algorithm, a sequence of zero value points in interferograms are picked out as seeds. A Voronoi figure surrounding these seeds is created, and the Voronoi figure segments each interferogram into several zones. The background of all points in these zones is set as the seed background, and a whole background is obtained when we combine these seed backgrounds. Numerical simulation and experimental analysis are conducted. Numerical simulation results show that the root-mean-square error (RMSE) of demodulation phase of the proposed algorithm is close to that of the method with real background. Experimental results show that the demodulation phase RMSE is reduced by 16% compared with the traditional low-pass filtering method.