For distributed memory parallel computers, broadcast operations are widely used in a variety of applications. In this paper, we propose an efficient algorithm for broadcasting on an all-port wormhole-routed 2D torus with arbitrary size. The underlying network is assumed to support only the dimension-ordered unicast. By taking the advantage of the all-port model and the distance insensitivity of the wormhole routing, the proposed algorithm can greatly reduce the number of message-passing steps. In addition, it can be proved to be depth contention-free. The performance study in this paper clearly shows the advantage of the proposed algorithm.