To meet occasional needs for automatic generation of power network single-line diagrams from the common information model（CIM） in power transmission network online theoretical line loss analysis， it is necessary for the big power network to automatically generate its network layout and ask for rather short solving-time. In view of the problems with automatic plating， namely， local optimum and the long solving time， the plating of single-line diagrams for power transmission networks is transformed into a quadratic assignment problem using the ant colony algorithm with 3-opt optimization. By taking into consideration potential parallel computation， local optimum and global optimum are chosen to update the pheromone while ignoring pheromone updating between ants. Simulation results show that the requirement of the power transmission network online theoretical line loss analysis is met by clear layout and short solving time.