The ice coating on the transmission line seriously affects the safe and stable operation of the power grid. It is of great engineering significance and application value to accurately monitor the ice coverage of the line. In order to accurately judge the icing thickness of transmission lines, a calculation model of equivalent icing thickness of strain tower lines under combined loads based on the icing tension monitoring system is proposed, in which the tension and angle of strain tower lines are monitored on-line as input parameters, the effects of non-uniform distributed loads and wind loads of strain insulator strings are taken into account, and the calculation model is improved by modifying gravity direction loads. Compared with the test data of Xuefengshan Natural View Ice Station and the operation data of monitoring terminal in the overlying ice section of the South China Power Grid, the accuracy of the calculation model is verified. Combining with the engineering practice, this paper has a certain guiding role in judging the icing situation of the strain tower.