The invention belongs to the field of power grid construction, relates to mid and low-voltage distribution network loss reduction, and particularly relates to an urban mid and low-voltage distribution network loss reduction method. The method comprises the following effective steps: various types of basic plan information in the city are firstly surveyed, analyzed and checked, wherein the basic plan information comprises load prediction data and substation information; according to the surveyed information, the wire section of the distribution line and transformers are regulated timely, wherein the wire section of the distribution line uses a main line of more than 80 mm<2> and a branch line of more than 55 mm<2> as a standard, and at least two transformers are in parallel connection for operation; according to the load prediction data, whether a loss reduction mode of balancing the three-phase load needs to be adopted for a three-phase load unbalance condition is judged; and if yes, a genetic algorithm is adopted for reactive power compensation and optimal configuration, and the purpose of urban mid and low-voltage distribution network loss reduction can be achieved.