Lightning seriously threatens the safe and stable operation of power system. With the continuous development of power system， the traditional line lightning protection assessment technology has been unable to meet the requirements of lightning risk assessment. Based on this， this paper uses the coefficient of variation method to analyze the data of lightning falling in recent years within 10 km of ten important transmission channels of 500 kV inter-provincial connection lines in central China， and finds that subsequent lightning return strokes have a great impact on lightning strike trip. At the same time， many circuit actual lightning trip faults are related to subsequent lightning return strokes. Accordingly， this paper proposes a flashover criterion model of insulator string considering subsequent return stroke. The influence of subsequent lightning return stroke on lightning resistance level of 500 kV transmission lines is analyzed. The results show that the lightning resistance level of 500 kV transmission line will be reduced by the follow-up return stroke， which is related to the power frequency voltage cycle， and the impact of the subsequent lightning return stroke on the lightning back-flashover resistance level is more significant than the lightning shielding resistance level. The conclusion of this paper can help improve the accuracy of lightning risk assessment of high-voltage transmission lines and provide help for lightning protection and early warning of high-voltage transmission lines.