Resolving the contradiction between trees and power transmission lines in time is an important work to ensure the safety of transmission line operation. While it is difficult to decide the trees trimmed time during the transmission line operation and maintenance, the development of the management information system for protecting distance between transmission lines and trees, which is used to auxiliary predict the date of pruning, is a way to solve the problem. Collected attribute data of transmission lines and trees, the corresponding database is established. Therefore, Visual Basic 6.0 is adopted to develop the software system. Based on the spatial coordinates of line towers, conductors, and tree crown, the three-dimensional mathematical model is established to calculate the spatial distance between the conductor and tree crown. According to the mathematical model, the trees growth rate of year, month and day, respectively, and the protecting distance standard between conductor and tree crown, the system could calculate the actual spatial distance between conductor and tree crown, and auxiliary predict the pruning date.