A novel method for predicting the short-circuit current of 220 kV sub-transmission network is proposed. First， the function relationship is built between the installed capacity of plant and its contribution to the short-circuit current of 220 kV bus in the 500 kV substation. Then it is pointed out that the sub-transmission network reaches its maximal capacity of power supply when 220 kV bus and 500 kV bus reach the equipment breaking capacity at the same time. The relationship is established between the maximal capacity of power supply of the sub-transmission network and the short-circuit current of 220 kV power grid. The validity of the model proposed is demonstrated by the comparison with an actual power network. Some conclusions obtained from an analysis of the calculation results provide reference to the transmission network equipment selection and the mode of power supply for the sub-transmission network.