Actual operation experiments show that invalid protection of parallel gaps often occurs when parallel gaps recommended by relative standard are used. New type of 500 kV parallel gap used in normal region has been developed, while in high altitude region, the size of new type 500 kV parallel gap cannot be used directly. In this paper, U_50% lightning impulse test, volt-time characteristic test, discharge visible corona and radio interference test of parallel gaps with different size are carried out in National Engineering Laboratory for UHV Engineering Technology (Kunming), the altitude of which is about 2 100 m, and the effectiveness of protection of parallel gap is analyzed; then the lightning performances with and without parallel gap are acquired by simulation. The result shows that, when the numbers of insulator of insulator string are 33 and 37, the crosswise lengths of electrode are respectively recommended as 800/800 mm and 850/850 mm, and after installing parallel gaps, the lightning performance of single circuit is reduced compared with no parallel gap, while the lightning performance of double circuit simultaneously flashover is improved, and the increased percentage is respectively 11.3% and 11.6%.