Abstract: A method of calculating the diameter of double-loop networks G（N；r，s） is presented based on step diameter method which is given by predecessors.A topologic structure of span-tree which using to study double-loop networks can be got.The properties of span-tree are studied and an algorithm of constructing span-tree is presented.Also a simple formula for expressing diameter d（N；r，s） of double-loop networks is presented.The structural model of span-tree is stimulated which makes use of C Sharp as programming language.The tight-optimal，almost-tight-optimal and k-tight-optimal results can be calculated for N is given random and 1≤r≠s＜N.

摘要： 基于前人提出的双环网络G（N；r，s）的分步直径求解法，提出了一个等价树直径求解方法，得到一个新的研究双环网络的拓扑结构－等价树；研究了双环网络等价树的性质并给出了等价树的构造算法；给出了双环网络直径d（N；r，s）的显示公式；利用C＃编程语言对等价生成树的结构模型进行了仿真实现；对任意给定的N，1≤r≠s＜N，可以计算出双环网络G（N；r，s）的紧优、几乎紧优、k紧优解。