The invention relates to the field of operating methods of measurement and diagnosis technologies, and more specifically relates to a method for mounting a current transformer at a grounding down lead of an operating lightning arrester. The method comprises the steps for confirming information of the lightning arrester, connecting a branch circuit with a current meter to the grounding down lead of the lightning arrester in parallel, disconnecting the grounding down lead of the lightning arrester, embedding the current transformer into the disconnected grounding down lead, recovering the grounding down lead, fixing the current transformer, and removing the parallel branch circuit. The method effectively solves a problem of getting an electric shock when mounting the current transformer on the grounding down lead of the operating lighting arrester. In addition, the method is simple and convenient to operate, safe, reliable and good in effect, can avoid power failure of the lightning arrester and improves the power supply reliability.