Based on 500 kV GIS withstand voltage test of 500 kV Mangkang substation with altitude 4300 m in central Tibet networking project，the determination of external insulation level of GIS withstand voltage test equipment at high altitudes，the selection of resonant circuit parameters and the influence of air corona on the test are discussed． The parameter configuration process of GIS AC withstand voltage test in high altitude area is proposed． Besides，to calibrate the external insulation level of test equipment by using the altitude correction method of external insulation of electrical equipment is preliminarily proved to be feasible． During the test，it is found that the corona of test circuit is very strong which leads to the sudden change of parameters of resonant circuit and causes the output voltage to fluctuate greatly． The research results provide new insights and references for the follow － up GIS AC withstand voltage test in high altitude areas．