Abstract: Based on the analysis of the similarities and differences between solving the shortest path problem and solving the longest path problem, an algorithm using the dynamic programming strategy was given to solve the longest problem in directed graph. The algorithm was applied to the treelength scaling network to solve the optimal treelength scaling problem. An optimization method using the treelength scaling triangle was proposed to optimize treelength scaling network. It reduced the complexity of the algorithm. The correctness and feasibility of the algorithm and optimization methods were verified through the experiments.