Aiming at the shape measurement requirements of nano-step objects, a partially coherent optical offaxis image plane interferometry system is designed and constructed. Phase unwrapping, as a key step of interferometry, requires fast speed, high precision and strong adaptability. In this paper, the characteristics of five common phase unwrapping algorithms are analyzed, and a fusion algorithm based on region segmentation is proposed based on the morphology characteristics of nanosteps. This algorithm solves the quality of each segment by combining the light intensity map with the tangent line, and uses the powerless least square method and the tangent method to phase expansion in the high and low quality regions respectively, so as to meet the requirements of real-time measurement and accuracy. The experimental results show that, compared with other algorithms, the proposed algorithm has high precision, fast solving speed, anti-interference to residual handicap, and has certain application value to the measurement of nanometer step topography under partial coherent light interference.