A phase unwrapping algorithm based on the solution of discrete Poisson equation is presented to retrieve the true phase of the object from wrapped phase. Algorithm model is constructed aiming at phase discontinuous and jumping caused by oddity points in interference pattern. Discrete cosine transform (DCT) is introduced to solve discrete Poisson equation which has Neumann boundary condition. Phase unwrapping results are obtained under least-squares method meaning. In comparison with the traditional algorithm, this method can effectively remove the discontinuous influence of shadow, blind spots, holes and noises, remove order jumping and wire drawing and resume true phase at the same time. Preferable experimental results are obtained in phase-shifting interference measurement.