With the accidents of 500kV over-line being attacked by the lighting increasing, on-line monitoring of over-voltage lines has become an important research topic at home and abroad. To analysis the character under different impact current of non-linear metal oxide varistor value(MOV), and to do the curve fitting of the 500kV metal oxide surge arrestor non-liner characteristic in a small current role, to derive the relationship between the terminal voltage of MOV and over-voltage online. Utilize ATP-EMTP to simulate equivalent model of metal oxide MOV under surge impact current, to verify the feasibility of the equivalent model. By the methods above, different characteristics of MOA effecting on the derivation of the over-voltage be analysis and thus provide the basis for monitoring overvoltage.