According to the existing capacity of emergency power supply in each local supply district and important customers’ needs for emergency power supply， by comprehensively considering the relationship between the reliability of power supply and utilities’ economy， the optimum model of emergency power supplies with minimum total cost as objective function is proposed based on such constraints as time and capacity etc. The optimal algorithm of emergency power supplies based on genetic algorithm is put forward. The model and algorithm proposed are proved by a numerical example and program based on geographic information system（GIS）. The study helps to reduce outage costs and improve the ability of urban electric network for emergency situation.