The influences of second order PMD on light pulse are discussed. The results by number simulation show that the second-order polarization mode dispersion (PMD) components associated with depolarization of the principal states can cause significant system impairments. By the same time the compensation method for second-order PMD are also discussed by theoretically and experimentally. The experiment results show that it is very effective to compensate second order PMD by using two stage PMD compensator.