Based on the background of Rudong 150MW offshore wind farm demonstration project, this paper expounds the principle of lightning impulse test and breakdown phenomenon of the offshore GIS equipment, the reason of breakdown is analyzed and the relevant preventive measures and suggestions are given. First of all, the principle of lightning impulse test is described, and then the test platform is built, the results show that both two different voltage degree GIS which are tested are breakdown in different parts. Through the inspection of the discharge points of the two GIS, it is found that the breakdowns in this test are reducible metal gap breakdowns, combined with experimental results and test environment, it is deduced that the breakdowns are due to the metal debris which are introduced in the transport and installation processes of GIS. At last, some relevant suggestions are given to prevent the breakdown and improve the test.