A semitrailer I beam comprises an upper wing plate, a lower wing plate and a web, wherein the upper edge of the web is aligned; the lower edge of the web has a higher left side and a lower right side and is provided with an arc-shaped transitional part; the upper wing plate is vertically arranged on the upper side of the web; the lower wing plate of which the shape is matched with that of the lower edge of the web is vertically arranged on the lower side of the web; straight lines to which the upper edges of the left part and the right part of the web and the upper edge of the web belong are mutually parallel; and the upper wing plate and the lower wing plate are welded with the web respectively. The semitrailer I beam is simple in structure and practical in functions; materials are saved on the premise of ensuring the strength of the I beam; and the manufacturing cost of the semitrailer I beam is reduced.