The utility model relates to an on-line monitor and an online monitoring system of an ultrahigh-voltage DC lightning arrester. The on-line monitor comprises a resistance chip, a counter loop, a current measuring loop, a field counter, a field current meter and an electro-optical conversion loop, wherein one end of the resistance chip is connected with a high voltage, another end is grounded, an input end of the counter loop is connected with one end, connected with the high voltage, of the resistance chip, an output end of the counter loop is connected with an input end of the current measuring loop, an output end of the current measuring loop is connected with the grounded end of the resistance chip, the field counter and the electro-optical conversion loop are accessed into a counter measuring loop, the field counter and the electro-optical conversion loop are driven by the counter measuring loop, and the field current meter is accessed into the current measuring loop. The on-line monitor and the online monitoring system of the ultrahigh-voltage DC lightning arrester adopts a passive mode, and transmits lightning arrester action signals through an optical signal mode; and the action times and action time of the lightning arrester can be recorded through the judgment processing of the optical signals, and the magnitude of the impact current can be obtained accurately.