330kV transmission line in the northwest region of China is much complex while pollution flashover is a major threat to the safe operation of the system. In this paper, experimental study on effect of contamination on electric field distribution of 330kV composite insulator is presented. High voltage test system of 330kV composite insulator is established according to the ratio of 1:1. Surface electric field and potential of the insulator is measured using GDC-100 optical fiber voltage electric field measuring instrument. Potential, electric field distribution of the two kinds of 330kV composite insulator are tested in different environment of clean, dry and uniform pollution layer. The test and simulation result is also compared.