The composite insulators are likely to have such defects as aging, damage and mandrel carbonization after a long-term operation, which could cause safety problem for its’ live-line maintenance. In this paper, taking the I-series composite insulators of a 500 kV transmission line as an example, the discharge characteristics of the composite insulators under different conductivity defects are obtained through impulse discharge tests, and the location and size limitation requirements of the conductivity defects are determined through the insulation coordination method of live-line working. The testing results show that when the conductivity defect exists at the high-voltage end, the impulse discharge voltage of the gap is lower, which has a greater impact on the safety of live-line working; In order to ensure the safety of live-line working, when the conductivity defect exists at the high-voltage end of composite insulator, the maximum allowable length of conductivity defect is 0.7m; when the conductivity defect exists in the middle of composite insulator, the discharge voltage is the lowest when defect exists at the position about 1/4 of the length of the insulator away from the high-voltage end, and the maximum allowable length of conductivity defect is 0.9m. The location and size limitation requirements of the conductivity defects of composite insulators determined in this paper can provide a technical support for the safety of live-line overhaul working of composite insulators on transmission lines and guide the safety detection of composite insulators before live-line working.