A device for manufacturing a seamless hat includes an assembling cutting device and a layer connecting device. The assembling cutting device includes an ultrasonic metal wheel, an ultrasonic metal platform, a feeding rubber wheel, and a feeding metal wheel. The ultrasonic metal wheel and the feeding rubber wheel are concentric and close to each other. The metal wheel is disposed at the bottom of the feeding rubber wheel. The ultrasonic metal platform is disposed at the bottom of the ultrasonic metal wheel. A top end of a tapered peripheral surface of the ultrasonic metal wheel has a sharp edge and a platform portion, which is disposed at a side away from the rubber wheel. Accordingly, the seam allowance cut pieces of the hat may not be initially connected to the edge of the hat. The assembling cutting device simultaneously proceeds the ultrasonic connection of the seam allowance cut pieces and the residue of the cloth is cut in order that the connection is uniform and to get more efficient.