An approximate analytical expression of the stress birefringence for Panda polarization-maintaining fiber (Panda-PMF) was deduced by using elasticity theory and complex function method. Based on this expression, the birefringence properties of this fiber were analyzed numerically by using the finite-element method. The results show that the birefringence expressed by core average stress mode is more accurate than that expressed by the core center stress mode and the error of the former is less than 0.18% compared with that of the analytic method. Further more, the birefringence degradation of the Panda-PMF induced by stress area mismatch was analyzed and proved by the related experiment.