The precision of phase extraction directly affects that of interferometry. The traditional fixed-step or equal-step length phase extraction algorithm must carry out phase calibration for the test system, but phase shift error is often introduced due to inaccurate phase calibration, which affects the precision of phase extraction. Therefore, this paper proposes a random two-step phase shift algorithm of K-order two-dimensional polynomial fitting background light (PFBL) to solve phase. This algorithm does not need to carry out phase calibration, and can solve the measured phase with only two frames of phase-shift interferogram when phase shift quantity, background light, modulation system and phase are unknown. When order number K value is greater than or equal to 2, it is determined by the simulation of K-order two-dimensional polynomial, the accuracy of the proposed algorithm for phase can be higher. At the same time, the algorithm robustness is analyzed by comparing the calculation accuracy of Gram-Schmidt orthogonalization (GS) two step phase-shift algorithm with the PFBL method. The results show that the PFBL method has good robustness in the interferogram phase shift, non-uniform illumination and noise, and the calculation accuracy of the PFBL method is obviously better than that of GS algorithm.