Variable　polarity　plasma　arc　(VPPA)　welding　has　shown　its　unique　advantages　in　the　welding　of　thick　aluminum　alloy　plate.　Pilot　arc　and　its　power　source　have　great　influences　on　arc　shape,　mechanical　properties　and　static　characteristic　of　arc　in　variable　polarity　plasma　arc.　By　means　of　high-speed　photograph,　pressure　sensor　and　signal　detection　system　based　on　industrial　computer,　the　rule　of　influences　of　pilot　arc　on　arc　shape,　arc　force　and　welding　process　are　systematically　analyzed.　It　is　found　that　during　the　reversed　polarity　period　of　VPPA,　the　arc　force　is　decreasing　due　to　the　shunting　action　of　power　source　of　pilot　arc,　which　can　easily　cause　such　phenomena　as　double　arc　and　decrease　of　arc　voltage.　The　experimental　results　have　important　theoretical　guiding　significance　to　the　stability　of　plasma　welding　process　and　to　the　welding　process　control.