The modeling of the SCADA system , which is based on the unit of bay is discussed in this paper. Bay is composed of a series of associated electric equipments, which is usually used in electronic system. Modeling in bay-level will greatly simplify the maintenance of SCADA system. Three modeling methods, searching SCADA data table, building model information table and unifying the two methods in bay-level are presented with their advantages and disadvantages. Then this paper discusses how to connect the graph equipments when plotting in bay-level. The application of modeling in bay-level in the SCADA system of Zhongshan Power Supply Bureau is illustrated.