A　new　environment-friendly　automatic　steel　alloyed　with　tin　element　Y20Sn　has　been　developed.　In　test,　new　steel　possess　good　machinability　and　mechanical　property,　which　reach　Chinese　National　Standard　of　corresponding　automatic　steel　GB/T8731-88.　In　steel,　the　shape　of　inclusions　is　mainly　spherical　or　spindly,　which　volume　is　small　and　distribution　is　evenly.　Such　style　of　inclusion　can　fairly　improve　the　steel　machinability,　and　doesn＇t　impair　its　mechanical　property　at　the　same　time.　As　the　main　inclusion　in　developed　automatic　steel,　MnS　inclusion　is　one　important　factor　which　influences　the　machinability　of　steel.　In　the　steel　alloyed　with　tin,　tin　exist　around　MnS　inclusions　or　are　attached　to　MnS　inclusion　will　influence　the　formation　mechanism　of　MnS　inclusion,　that　is　the　important　reason　why　tin　element　influence　machinability　of　developed　automatic　steel　Y20Sn.