The invention discloses an online check method for an online monitor of a lightning arrester. The online check method for the online monitor of the lightning arrester comprises the following steps: when a milliammeter of the online monitor of the lightning arrester is detected, connecting a current measurement device with the online monitor of the lightning arrester in parallel, and reading the current values of the milliammeter and a current measurement unit; judging whether the milliammeter of the online monitor of the lightning arrester is in fault; when an operation counter of the online monitor of the lightning arrester is detected, connecting a high voltage generator with the online monitor of the lightning arrester in parallel, and inputting high-voltage current; and judging whether the operation counter is in fault. As the online check method disclosed by the embodiment of the invention can detect multiple online monitors of the lightning arrester on the site of a transformer substation, and periodically detect all online monitors of the lightning arrester, the safety of the transformer substation can be effectively improved, and as all the online monitors of the lightning arrester can be online detected within one operation at the transformer substation, the online monitors of the lightning arrester do not need to be removed or conveyed anymore, so that the work efficiency is effectively increased, and the manpower and material costs are greatly saved.