Bushing is an important part of high voltage transformer, GIS and other core equipment. The external insulation design of bushing mainly depends on the air gap distance required by the switching impulse discharge voltage and the creep distance required by the polluted external insulation. With the increase of voltage level and altitude, the bushing length will become too long to manufacture according to the traditional design method. The discharge characteristics of bushing external insulation are affected by the core. In this paper, the external insulation gap discharge tests of 550 kV composite bushing are carried out by using bushing with core, hollow bushing and single grading ring respectively. The results show that the discharge voltage of the bushing with core is higher than that of the hollow bushing, and the discharge voltage of the hollow bushing is almost the same with that of the single grading ring. It shows that the bushing core is beneficial to increase the discharge voltage. The air gap distance designed according to traditional method which only depends on the discharge voltage of the single grading ring has great space for optimization. The research results provide a technical basis for the design of external insulation length of bushing, which is conducive to reducing the bushing length, especially reducing the difficulty for manufacturing bushing applied to high-altitude.