Abstract: A new method to research optimal parallel routing of unidirectional double-loop networks (DLN) is presented based on Cartesian coordinates. Firstly, the ditributed model of copy nodes is obtained under the research on the distributed rules of the copy nodes of nodes on both X axis and Y axis in the minium distance diagram(MDD). Using this ditributed model of copy nodes, the optimal parallel routing method is provided, as well as calculating method of wide diameter of DLN. Secondly, the parallel distance diagram(PDD) of DLN which is expanded by MDD is designed and simulated. Based on the PDD, a method to find two shortest parallel pathes quickly without any repeating node was proposed. At last, the simulation of the distribution of wide diameter of DLN is given. The result of simulation indicates that the characteristic of wide diameter distribution is reposeful undulation. Compared with traditional routing method, optimal parallel routing method improved the transition efficiency of DLN obviously based on the simulation results.