A mold-concrete composite crossbeam includes a H-steel composed of upper and lower flanges and a web connecting them; a concrete member formed in a length direction of the H-steel to expose the upper flange out but bury the lower flange and the web partially; deck plate supports installed to both upper side edges of the concrete member to support a deck plate placed thereon; a reinforcing steel bar installed around the H-steel; a mold including a bottom detachably coupled to a lower end surface of the concrete member, and sidewalls formed in parallel to both sides of the bottom and detachably coupled to both end sides of the concrete member; a lateral reinforcing member coupled across both sidewalls; a side reinforcing member installed to an outer side of the sidewalls in a length direction; and bottom reinforcing members installed across a lower surface of the bottom.