A welding device for welding and transmitting belts includes a feeding mechanism, a positioning mechanism, a welding mechanism, and a control unit. The feeding mechanism is used for storing and transmitting the belts to the positioning mechanism. The positioning mechanism receives and positions the belts. The welding mechanism includes a cylinder assembly and a welding assembly connected to the cylinder assembly driven by the cylinder assembly in a vertical direction and in a horizontal direction, whereby the welding assembly is horizontally and vertically moved to the positioning mechanism. The control unit controls an operation of the feeding mechanism, the positioning mechanism, and the welding mechanism.