In urban medium-voltage distribution network with whole cable laying, the ring of cables formed by ring main unit (RMU) is widely used. The appropriate position and number of RMU in a ring can improve the network structure and reduce main cable breaking. Therefore, a new model is proposed to optimize the position and number of RMU and the ascription of common load between rings with the consideration of geographic information. According to the characteristics of the new model, genetic algorithm with a multi-information matrix encoding strategy and the corresponding genetic manipulation is presented. The validity and practicality of the proposed model and algorithm is illustrated by the simulation result of an actual example.