Accurate and efficient phase decoding algorithms play an important role in noncontact optical phase-measurement. This paper develops a new phase-shifting decoding algorithm based on analysis of the traditional method. At first, four-step phase-shifting method is used to get the unwrapping phase, The residues theory is used to check the residues. Then a new reliability ordering map is used to guide to check rest of the residues. Low-pass filter algorithm is used to get the optimum check results. At last we place the branch cuts to balance all the residuals and decode the whold image by flood algorithm. Experimentations show that the proposed optical decoding algorithm is effective and robust.