In order to study the impact of installing a new type of arrester in 110 kV over-head transmission lines on the electric field distribution of composite insulators， according to the actual structure of transmission towers， the new arrester， composite insulators，grading rings and fittings in the 110kV overhead transmission lines， a three-dimensional fi-nite element computational method of time-harmonic electric field is adopted to establish the computational model of the electric field strength in composite insulator string with and without the new arrester， by analyzing and comparing the electric field distribution of com-po site insulators and grading ring's surface before and after installing the new arrester， a conclusion can be drawn that installing the new arrester helps to increase the uniformity of electric field on the surface of composite insulators， and to improve the effect of voltage-sharing and have a positive impact.