The invention relates to a multifunctional auxiliary external fixing instrument for supracondylar fracture of humerus in children and belongs to the field of medical instruments. The multifunctional auxiliary external fixing instrument mainly comprises a rotary folding frame assembly, a front splint, an inner splint, an outer splint, and a connection hose; the rotary folding frame assembly comprises a front arm support plate, a rear splint, a special ball hinge assembly, an arc link-handle assembly, the front arm support plate and the rear splint are connected through the special ball hinge assembly, and the included angle between the front arm support plate and the rear splint is adjusted and stabilized through the arc link-handle assembly. The special ball hinge assembly comprises a ball hinge seat, a ball hinge rod, and ball hinge head locking bolts. The arc link-handle assembly is composed of a threaded arc link, a cylindrical rotary pin, a Y-shaped insert rod and two convex nuts. The multifunctional auxiliary external fixing instrument for supracondylar fracture of humerus in children can provide assisted traction reduction for supracondylar fracture in children, is quick to fix and convenient to assemble and disassemble, can prevent cubitus varus or valgus deformity, can provide effective physiotherapy for the fracture, and can assist the affected elbow in performing functional training.