This　paper　presents　a　motion　planning　algorithm　for　automatic　parallel　parking　under　given　parking　environment　and　arbitrary　initial　posture,　which　divides　the　parking　process　into　two　stages,　that　is,　from　any　initial　posture　to　the　target　posture　through　the　intermediate　posture　determined　by　duplex　motion　planning　strategy.　Moreover　we　employ　the　shortest　path　method　of　motion　planning　for　each　stage　in　the　whole　parking　process.　Simulation　results　show　that　the　algorithm　complete　the　automatic　parallel　parking　under　given　parking　environment　and　arbitrary　initial　posture　effectively.