The　finite　element　method　is　adopted　to　solve　the　Reynolds　equation　of　spiral　seals,　and　two　dimensional　pressure　distribution　of　mechanical　seals　is　presented.　The　seal　dynamic　properties,　such　as　stiffness　coefficients,　damping　coefficients,　torque　stiffness　coefficients　and　torque　damping　coefficients,　are　analyzed.　The　dynamic　properties　of　the　herringbone　spiral　seal　are　calculated,　and　the　influences　of　the　rotating　speed　and　clearance　on　the　dynamic　properties　of　the　seal　are　studied.　The　calculation　results　show　that　if　the　clearance　of　the　mechanical　seal　is　large　enough　(＞10　μm)　the　dynamic　influences　of　the　seal　liquid　can　be　neglected,　and　the　damping　coefficients　and　torque　damping　coefficients　hardly　change　with　the　variation　of　rotating　speed,　but　change　greatly　with　the　seal　clearance.　At　high　rotating　speed　the　stiffness　coefficients　and　torque　stiffness　coefficients　increase　at　first　and　then　decrease　with　the　increasing　of　seal　clearance;　if　the　rotating　speed　is　low,　the　stiffness　coefficients　and　torque　stiffness　coefficients　decrease　with　increasing　of　seal　clearance,　and　within　some　regions　the　stiffness　coefficients　and　torque　stiffness　coefficients　may　even　be　negative,　which　leads　the　mechanical　seal　system　to　instability.