Abstract:Let uniform electric field, whose electric intensity is E → 0 which is composed of thunder, rain and earth, ad reference sys2 tem to solve electric field E → around the top of lightning rod, the relativity between electric field of lightning rod and electric field of earth surface is firstly arrived at - E≥1 ×103 E0. According to this equation, lightning rod through corona discharge can control electric in2 tensity of earth surface as under 0. 1 percent of air disrup tive field intensity, which p rovides theoretical exp lanation for that p robability, whose scope which is p rotected by lightning rod is hit by thunder, is less than 0. 1 percent. This exp lanation broadens the human recog2 nition of the actingmechanism of lightning rod from the angle of electric field and intensity.