Lightning striking on power system may lead to severe accident and huge losses, which is the reason why line arresters are needed. During operation, the electrical field concentration in line arrester may lead to ablation and accelerated aging of insulation materials, and furthermore, cause mal-operation. A type of line arresters with internal series air gap (IGLA) is chosen by this study, which is distinguished from the previously used metallic oxide arrester, with better characteristic on anti-flashover when icing, but with worse characteristic on anti-flashover when polluted. This paper aims at studying the probable variation of power-frequency electric field of this new kind of IGLA under polluted and icing situation, and making further analysis on the reason why the electric field strength change like this.