The mechanical model of icing and radian variation of conductors is built， the installing construction of force sensor is designed， and the on-line monitoring system of transmission line conductor icing is developed based on GSM SMS. The results of field operation show that the on-line monitoring unit placed on every transmission line tower can timely measure the gravity of iced conductors， the deflection angel of insulators， the deviation angel of insulators for wind， gallop frequency of conductors， the wind velocity and etc. It can send the gathered information to the municipal monitoring and controlling center through SMS. The conductor icing status is also analyzed based on the mechanical model built firstly， the alarming message of clearing conductor icing is sent out in time， and the safe operation of transmission lines is effectively guaranteed.