The invention discloses a truss sheet unit of a steel truss girder, a steel truss girder structure and a mounting method thereof, relating to the design and the construction techniques of heavy-duty and large-span bridges. The steel truss girder structure consists of a plurality of the truss sheet units which are welded into a whole, wherein the truss sheet units comprise upper chord rods, low chord rods, first web rods and second web rods; the upper chord rods, the lower chord rods, the first web rods and the second web rods are welded into a rectangular frame body; two across corners of the rectangular frame body are connected by a diagonal rod; two ends of each upper chord rod further extend to form two upper connecting rods; two ends of each lower chord rod further extend to form two lower connecting rods; simultaneously, first oblique connecting rods which extend downwards are arranged among the first web rods and the upper connecting rods; and second oblique connecting rods which extend upwards are arranged among the second web rods and the lower connecting rods. When the steel truss girder is assembled, the whole truss sheet units are lifted each time; furthermore, the truss sheet units occupy two internodal lengths; the lifting times are less; the filed assembly is simple; the quantity of high-strength bolts is reduced; and the influence of the screwing of the high-strength bolts on a construction period is decreased.