To research the characteristics of AC discharge under extreme high altitude, low pressure conditions, this research built up a test platform to simulate AC discharge of long gaps under low pressure and tested its gas tightness and discharge performance. This platform is consisted of a vacuum test chamber, vacuum pump, vacuum gauge, AC power source, high-speed camera, etc.; pressure and gap length are adjustable within 23 Pa～100 kPa and 600 mm respectively. The results show that, this platform meets the requirements for simulating AC discharge of long gaps in 20 km and above altitude, and has good gas tightness and discharge performance for the research of AC discharge characteristics which test period is short. These results have reference value for researching the characteristics of AC discharge of long gaps under low pressure.