Abstract: Selection of route is the most important aspect in the design of power transmission line and directly affects the total project cost and the reliability of operation. The model of global optimal route searching was constructed according to the characteristic of environment and the connective graph was processed. Then, the selection strategy and the pheromone updating method of the basic ant colony algorithm (ACA) was improved. For some content, the influencing factors on the choice of the optimal route such as geological conditions, hydrological conditions and forest land were added to the new algorithm, and the improved ACA was applied directly to search the global optimal route on the model constructed above. Simulation results showed that the method can exactly search out the global optimal route actually.