The one-line diagram for a distribution network, as an electrical wiring diagram, uses the specified graphic symbols and equipment connections to visualize the distribution network, significantly improving the efficiency of distribution network management. Aiming at the actual needs for automatic generation of one-line diagrams for a distribution network, this paper provides an automatic generation method based on the trunk-branch line model. The method first creates the trunk line and branch lines and calculates the branch layout direction; then plans the initial branch layout and performs partial stretching; and finally enables branch partial contraction and automatic routing. It has been verified that this automatic generation method can automatically generate the one-line diagrams that meet the requirements of accuracy and aesthetics.