Provided is an outer hanging type synchronous gear transmission mechanism. A horizontal supporting plate is arranged in a box body, an end plate is arranged at one end of the box body, and an opening is formed in the other end of the box body. Three coaxial roller spindle mounting holes are respectively formed in the supporting plate in the box body and the end plate of the box body. Three roller spindles are arranged in the roller spindle mounting holes respectively, so that the motor speed reducer connecting ends of the three roller spindles are located at the opening end of the box body. A first right-handed rotation bevel gear, a left-handed rotation bevel gear and a second right-handed rotation bevel gear are respectively located at the position of the opening end of the box body and are fixed to roller spindles through keys respectively. The left-handed rotation bevel gear is located between the first right-handed rotation bevel gear and the second right-handed rotation bevel gear and is meshed with the first right-handed rotation bevel gear and the second right-handed rotation bevel gear at the same time. The outer hanging type synchronous gear transmission mechanism has the advantages of being good in manufacturability, assembly and balance and convenient to disassemble, reducing working difficulties of assembling personnel and easily ensuring that hole shafts have not clearance fit so as to ensure high precision transmission.