A　laser　beam　joining　an　Al-on-steel　overlap　configuration　of　5052　aluminum　alloy　and　Q235　steel　was　conducted　with　Sn-30Zn　filler　wire.　The　influence　of　processing　parameters　on　weld　appearance　was　investigated.　The　microstructures　and　mechanical　property　of　the　joints　were　analyzed.　The　results　show　that　Sn-Zn　filler　wire　could　well　wet　5052　aluminum　alloy　and　the　steel　under　heating　of　defocusing　laser.　Good　weld　appearance　was　obtained　by　suitable　processing　parameters.　The　solid　solution　interface　was　formed　at　the　aluminum　alloy　side　and　intermetallic　compounds　(IMCs)　interface　was　formed　at　the　steel　side.　With　increasing　laser　power,　inter　granular　penetration　of　the　Al　side　aggravated,　and　the　thickness　of　the　IMCs　layer　at　the　steel　side　increased.　It　was　found　that　all　of　the　joints　fractured　in　the　seam　according　to　the　results　of　the　tensile　tests.　The　intergranular　penetration　and　IMCs　did　not　reduce　the　mechanical　property　of　the　joint.　The　maximum　value　of　fracture　load　reached　312　N/mm.　The　fractured　surface　was　characterized　as　a　ductile　fracture.　The　influence　of　processing　parameters　on　the　microstructures　and　mechanical　property　was　discussed.