Power transmission line icing has harmful effects to the safety and stability of power grids, so monitoring the icing process is very important for power grids system operation. At present, the icing process mechanical model based on dynamic data of tension and angle has some errors when it is actually applied in monitoring system. According to this problem, this paper proposes an icing monitoring model of strain tower with multiple input and single output. It uses dynamic tension and angle data as the input, and the equivalent icing thickness as output. In order to identify the final model and calculate conveniently, the traditional system identification method is used by Matlab System Identification Toolbox. Simulation results show that the final icing model is able to estimate the icing thickness accurately in the field condition and provides a novel method for monitoring the power transmission line.