Segment-dividing model is appropriate for research on PMD. Mathematical deduction is made to change it into iterative model. It is easy to find that the new model gives clear meaning in physics and reveals the process of PMD accumulation in fiber. It is applicable for being transplanted to analysis on PMD. Theoretic analysis gives the reason for error in Numerical calculation. The iterative method is put forward for practical numeration. It gives instruction for PMD compensation. Calculation is curtailed in the new model.