The　polarization　mode　dispersion　(PMD)　of　the　optical　fiber　transmission　system　is　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　are　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.