The invention relates to an anti-highfield-interference transmission line electric megameter and a testing method thereof. A control computing unit sends a signal to control a motor to drive an electric generator to run; a direct current output is generated; an insulation resistor is tested by the direct current output with output protection; data are collected by a sampling unit, and are transmitted to the control computing unit through an anti-interface low-pass filtering unit to be computed; and the data are displayed by a display unit. Safety of engineering construction and low output current which can not resist interference currents under highfield interferences are synthetically considered to achieve most accurate measuring results.