A handle of assembly tool with an adjustable center of gravity includes a tongue piece and a tube that are connected to a functional part in the tool. A through hole is formed in the tongue piece. First and second plates are arranged on the tongue piece. The first plate is provided with a first buckle, a first slot, a first fixing hole and a first groove; the second plate is provided with a second buckle, a second slot, a second fixing hole and a second groove; a fixing part is formed in the first and second slots; a locking part is formed in the first and second grooves; the tube is connected with the first and second fixing plates, and a gasket is formed at the other end of the tube with a locking rod passed through; a weight unit is connected with the locking rod.