Energy　balance,　pressure　balance　and　temperature　of　keyhole　were　analyzed　based　on　the　assumption　that　the　keyhole　is　a　Paraboloid　of　revolution.　The　keyhole　depth　and　width　were　calculated　by　numerical　iterative　method.　The　calculated　results　accord　with　those　of　the　actual　experiments.　Then　the　keyhole　principle　fitted　for　low　speed　laser　welding　on　the　assumption　that　the　keyhole　with　cylinder　was　developed　to　a　high　speed.