A synchronous ultrasonic metal welding device with double-heads has two sets of ultrasonic welding mechanisms which are installed side by side on a middle frame (18). The middle frame (18) is composed of assembling flanges (19) and two sliding stop rods (11) which are in parallel each other. The ultrasonic welding mechanisms are located on the assembling flanges (19) which are connected with driving devices (3) respectively. The driving devices (3) drive the ultrasonic welding mechanisms moving toward each other and welding synchronously. By using the welding device of the invention, multi-layer metal pieces of the power battery can be double-sided welded simultaneously by way of spot welding or roll welding, so the work efficiency can be improved, and the welding layer can be strong in one time.