In order to overcome the slow convergence of the segmentation method based on the Chambolle's dual iterations to achieve high precision segmentation, we proposed an improved dual iteration of Chambolle to solve one of the sub-problems arising from the two-phase piecewise constant Mumford-Shah model. By using the LFA of the Chambolle's dual iterations and a modified dual iteration respectively, we have proved that the proposed algorithm is very reasonable. Experiment results have shown that, for the two-cell image and the dot256 image, when the accuracy of the termination criterion is higher, the number of iterations of the proposed algorithm is less, and it converges faster.