The location and supply area of 110 kV substations are the crux of the coordinated planning between the main system and distribution network. A concept of the coordinated factor and the coordinated planning model is presented， with emphasis on the global balance of system operation. First， the basic properties of substation are analyzed， based on which the properties of operation rate and correlation degree are used to construct the load weight coefficient. Then a bound circle is adopted to delineate the optimization area. With the load weight coefficient as the weight， the locations of substations are optimized using the improved weighted Steiner minimum spanning tree theory. Finally the capability and supply region of the substations can be determined. Actual examples show that the proposed coordinated planning is obviously better than those by traditional models.