The invention relates to a sensing technology based online open-type lightning arrester monitoring device and a monitoring method. The device comprises an open-type current transformer, a mounting bracket and a monitoring host; the open-type current transformer and the mounting host are mounted at two ends of the mounting bracket respectively; the open-type current transformer sleeves a lightning arrester and is in communication connection with the monitoring host; the open-type current transformer acquires total-current information on the lightning arrester in real time and transmits the total-current information to the monitoring host in real time; the monitoring host analyzes the total-current information to obtain a resistive current peak value, compares the resistive current peak value to a set threshold, and gives an alarm if the resistive current peak value exceeds the threshold. Compared with the prior art, the monitoring device and the monitoring method have the advantages that real-time online monitoring of the lightning arrester is realized effectively, and the electric mounting problem troubling the online lightning arrester monitoring device is solved really and effectively.