The earthing resistance is one of important technical parameters to reflect the earthing effect of lightning protection system; however, the testing of earthing resistance is closely related with its environment. According to the theory of electric capacity and electromagnetic fields, the influencing mechanisms of external interference sources, such as high voltage transmission line, television transmitting station or base station, underground cable, tracks etc., on earthing resistance testers are studied. The results show that the external interference sources seriously interfere the measuring of earthing resistance testing devices through capacitive coupling, resistive coupling, and inductive coupling. On this basis, it is concluded that the shield method and the equipotential bonding measuring method are effective for preventing interference on earthing resistance testing.