There are three typical structure modes in 220 kV district grid. The impact of the short circuit impedance of a 500 kV main transformer on the power supply capability of the three types of 220 kV district grids is studied. The results show that the power supply capability of a 220 kV district grid can be effectively improved by increasing the short circuit impedance of the 500 kV main transformer. So it is recommended that a new 500 kV transformer substation should be equipped with transformers with high impedance at its high-and middle-voltage sides. According to the practice and current technical level of the equipment manufacturers， 20% is a feasible impedance percentage.