The　welding　stress　and　strain　were　investigated　using　the　finite　element　method　combined　with　the　experiments.　The　research　results　showed　that　the　metal　in　the　melting　pool　is　melted　in　the　frame　of　area　with　compressive　plastic　strain,　and　cooled　down　also　in　the　frame　of　the　area　with　compressive　plastic　strain.　The　tensile　plastic　strain　is　produced　in　the　weld　when　it　is　in　the　state　of　＇mechanical　melting＇　during　the　cooling　process.　But　in　the　residual　state,　the　compressive　plastic　strain　still　remains　in　the　weld.　The　peak　value　of　longitudinal　tensile　stress　in　the　weld　of　titanium　alloy　is　always　lower　than　its　yield　strength　at　the　appropriate　temperature　during　the　cooling　process,　this　situation　is　kept　until　the　residual　state.