The invention discloses a power distribution network structure on-line loop closing flow optimization method. The method comprises the following steps: step I, topology analysis is performed to generate topology data for calculation; step II, closed loop flow calculation is performed to calculate the flow distribution before loop closing and after loop closing; step III, loop closing flow analysis is performed, and analysis and evaluation data is formed; step IV, and graphic display is performed to show the loop closing flow analysis and evaluation result to users in graphics or forms manner. According to the invention, optimization is performed on the power distribution network structure by adopting an on-line loop closing flow optimization algorithm, so that the method can be used to effectively guide dispatchers to choose the best time to perform the loop closing operation to ensure the success of the loop closing operation, the equipment safety can be ensured, the uninterruptible power supply to a power user can be ensured, and the power supply reliability can be improved.