Abstract: Double loop network is one of the most important topologies used in interconnection networks. The traditional methods of optimal routing for this topology are not optimal because they have not considered the relationship of shortest paths which have the same source node. A new concept, the link prior shortest path, is defined first. In this way, the shortest path between any two nodes is one and only. Also the link prior shortest paths which have the same source node can be expressed by a formula and can be calculated easily. In practice, a source node can record and store those "abnormal" nodes, which link prior shortest path from that source node can’t be obtained by adding one link to its next node’s link prior shortest path from the same source node, and their link prior shortest paths. Using these results, a source node can find the link prior shortest path to any target node with much fewer arithmetic operations than traditional ways.