Branch flow model is formed when the connection relationship of branch is fixed. So it cannot represent the relationship of voltage, current and flow when there is a switch and the state of the switch is uncertain. To solve this problem, an improved branch flow model is proposed in this paper. The model is improved by adding virtual nodes and branches. It can represent two states at the same time. When the switch is on, the functional relationship of voltage, current and flow of the branch can be represented. When the switch is off, the non-connected relationship can be represented. Then a distribution network reconfiguration model based on improved branch power flow is established. It can be relaxed and solved by second-order cone optimization. Finally, IEEE 33 node test system is adopted to verify the effectiveness of the improved model.