A steam iron including an ironing unit that includes an ironing board member, a heating tube which is mounted fixedly in said ironing board member and which has left and right heating tube sections, and a housing which is mounted on top of said ironing board member, said steam iron being characterized in that said ironing board member is formed with: a pre-heating channel that is formed between said left and right heating tube sections of said heating tube for guiding water introduced therein to flow rearwardly to be pre-heated by said left and right heating tube sections; two heating channels that are in fluid communication with a rear end of said pre-heating channel, that flank said pre-heating channel, and that extend forward and are disposed respectively and substantially over said left and right heating tube sections for heating the pre-heated water coming from the pre-heating channel such that the pre-heated water is boiled and vaporized into high-temperature steam; a steam channel that is in fluid communication with front ends of said heating channels, and that has two divisional sections flanking and being respectively adjacent to said heating channels and extending rearward for spreading out the steam which is pressured forward thereinto from said heating channels; and a plurality of spaced-apart steam vents that are in spatial communication with and arranged along said divisional sections of said steam channel, such that the steam in said steam channel flows to escape from the ironing unit through the steam vents, further characterized in that said ironing unit further includes a linking tube connected to said housing, said housing having a connecting tube portion that is connected to said linking tube for introducing water from said linking tube into front end portion of said pre-heating channel, said steam iron further including a water tank unit that is connected to said linking tube and that is operable to pump water stored therein into said linking tube, further characterized in that said iron plate portion is further formed with a retention groove that is in fluid communication with said pre-heating channel and that is disposed under said connecting tube portion for receiving water flowing from said linking tube to said pre-heating channel. a pre-heating channel that is formed between said left and right heating tube sections of said heating tube for guiding water introduced therein to flow rearwardly to be pre-heated by said left and right heating tube sections; two heating channels that are in fluid communication with a rear end of said pre-heating channel, that flank said pre-heating channel, and that extend forward and are disposed respectively and substantially over said left and right heating tube sections for heating the pre-heated water coming from the pre-heating channel such that the pre-heated water is boiled and vaporized into high-temperature steam; a steam channel that is in fluid communication with front ends of said heating channels, and that has two divisional sections flanking and being respectively adjacent to said heating channels and extending rearward for spreading out the steam which is pressured forward thereinto from said heating channels; and a plurality of spaced-apart steam vents that are in spatial communication with and arranged along said divisional sections of said steam channel, such that the steam in said steam channel flows to escape from the ironing unit through the steam vents, further characterized in that said ironing unit further includes a linking tube connected to said housing, said housing having a connecting tube portion that is connected to said linking tube for introducing water from said linking tube into front end portion of said pre-heating channel, said steam iron further including a water tank unit that is connected to said linking tube and that is operable to pump water stored therein into said linking tube, further characterized in that said iron plate portion is further formed with a retention groove that is in fluid communication with said pre-heating channel and that is disposed under said connecting tube portion for receiving water flowing from said linking tube to said pre-heating channel.