A manufacturing method of flat-plate heat pipe includes steps of: providing a first board body, a second board body and a capillary structure; selectively affixing the capillary structure to the first board body or the second board body; overlapping and mating the first and second board bodies with each other and sealing the open peripheries of the first and second board bodies to form a flat tubular main body with a reserved air-sucking and water-filling section; and vacuuming the flat tubular main body and filling working fluid into the flat tubular main body and sealing the air-sucking and water-filling section. By means of the manufacturing method of the flat-plate heat pipe, the flat-plate heat pipe can be formed with a thin thickness. After the flat-plate heat pipe is thinned, the flat-plate heat pipe still has a complete vapor passage.