The polarization mode dispersion (PMD) of the optical fiber transmission system has been the main factor to limit the transmission capacity and distance of the system. The degree of polarization (DOP) of optical signals is related to the PMD of the system. The expression of the output pulse Stokes vector is derived based on Jones matrix model proposed by A. Orlandini. The basic properties of degree of polarization of optical pulse have been investigated. It is shown that the degree of polarization of optical signals changes with polarization state of the input, and reaches its maximum at the position of the principal state of polarization; it decreases with the increase of the PMD magnitude; And it decreases also with the increase of absolute value of the initial pulse chirp coefficient, but has no relation with the sign of chirping coefficient.