The present invention discloses an electric transmission line electromagnetic field intensity monitoring system. The system comprises an electromagnetic field intensity monitoring sensor module, a network module, a CPU module, a data storage module, a data receiving module and a data display module; the electromagnetic field intensity monitoring sensor comprises an electromagnetic field intensitysensing module, an A/D conversion network and a DSP processing circuit; and the A/D conversion circuit, the DSP processing circuit, the network module, the CPU module, the data storage module, the data receiving device and the data display module are connected in order according to the sequence. The present invention further discloses an electric transmission line electromagnetic field intensity monitoring method. The method performs monitoring for the local electromagnetic field intensity of the transmission line tower to determine whether the transmission line is overvoltage or not and receives transmission line local electromagnetic field data in a full time phase by taking the tower as a monitoring point to accurately determine the frequency of the overvoltage of the line. The electrictransmission line electromagnetic field intensity monitoring system and method provide reliable data support for the lightning protection measures of the transmission line.