This paper studies the insulation breakdown fault of a 35kV ring main unit busbar, analyzes the cause, nature and influence of the fault, and puts forward improvement measures and prevention suggestions. Through on-site inspection and fault recording analysis, it is found that the main causes of the fault are that the insulation busbar installation process does not meet the standard requirements, and the conductor contact surface is not good for a long time, resulting in insulation breakdown and short circuit accidents. The research of this paper has certain theoretical and practical significance for ensuring the safe and stable operation of 35kV ring main unit and improving the reliability and efficiency of power system.