Adding an internal down conductor provides a new way for lightning protection of the wind turbine, but it is necessary to analyze electromagnetic interference on internal cable line when lightning current flows through the down conductor. Circuit models of blades, tower and grounding system were established in EMTP to analyze current distribution characteristics of the down conductor and the tower. Voltage difference between the down conductor and the cable line and the average electric field were calculated which takes coupling effect into consideration. Safety distance between the down conductor and the cable line was presented and effects of current wave front time and soil resistivity were discussed. Research results show that: most of the lightning current flows through the down conductor and voltage difference between the down conductor and the cable line can be up to several MV which leads to side flashover easily. Voltage difference has a polarity reversal and average electric field firstly decreases and then increases with the decrease of height. Shortening equipotential bonding distance could reduce voltage difference and the average electric field effectively. Decrease of current wave front time or increase of current amplitude results in a rise of safety distance between the down conductor and the cable line while effect of soil resistivity is opposite. It is essential to set equipotential bonding distance and electrical safety distance properly when internal down conductor is adopted.