The problem of heavy ice covering transmission lines can be solved effectively by the development of DC deicer. When one of double-transmission lines hung on common-tower is operating in deicing mode， the other transmission line will have some influence on operation of the deicer because of electromagnetic and static induction between the double-transmission lines. Some cases have been simulated by the close loop simulation system which comprising a real-time digital simulator（RTDS） and a real control and protection equipment of deicer. Simulation results show as follows: DC deicer has few influence on operation of the common-tower transmission lines; the common-tower transmission lines will influence on the 50 Hz protection of DC deicer， and in order to get rid of it， the 50 Hz protection logic should be improved or increase act threshold; Trip of AC breaker of the common-tower transmission lines will impact on ground current of the deicer， and the action setting value of ground current protection should be increased properly to avoid mal-operation; Appropriate deicing method should be selected when a common-tower double-transmission lines operates in deicing mode.