The method of Poincare Sphere to measure the polarization mode dispersion (PMD) has been studied in the paper. Based on the statistical character of the output states, a simple and practical arithmetic to calculate the differential group delay (DGD) with the output states of polarization was put forward. With this arithmetic, the PMD of a 11 km long single mode fiber and a 1.64 m long polarization maintain fiber have been calculated, and the feasibility of the arithmetic is confirmed.