Abstract: The combined air gap of live-line work is composed of transmission lines, linemen and towers. The breakdown voltage of the combined gap in live-line work reflects the dielectric strength of the combined gap, which is of great safety significance for linemen. We built a discharge physical model that could describe the discharge development process of the combined air gap and calculate the gap breakdown voltage based on the observation results of the discharge physical processes of the rod-rod-plane combined gap and the rod-sphere+rod-plane combined gap under the positive switching impulse. The established physical model and the Rizk model were used to respectively calculate the discharge characteristics of the combined gap obtained in the experiment, it was found that the average error in calculating the discharge characteristics of the rod-rod-plane combined gap was 7.10% while the max error was 15.58%, and the average error of the rod-sphere+rod-plane combined gap was 5.83% while the max error was 13.10%. Compared to the Rizk model, the model proposed in this paper can be adopted to calculate the discharge characteristics of the combined gaps more accurately, verifying the accuracy of the physical model. In addition, generalization experiments and control experiments were conducted to analyze the effect of model generalization ability and influencing factors on breakdown voltage.