With　the　development　of　the　power　grid　at　high　altitudes,　more　and　more　transmission　and　transformation　projects　are　being　implemented　at　altitudes　close　to　5000　meters.　As　the　altitude　increases,　the　air　pressure　decreases,　resulting　in　a　decrease　in　the　insulation　performance　of　the　air　gap　and　requiring　corresponding　adjustments　to　the　insulation　structure　of　power　equipment.　This　paper　investigated　the　AC　breakdown　characteristics　of　air　in　non-uniform　electric　fields　at　pressures　of　50100　kPa.　The　relationship　between　maximum　breakdown　field　strength　and　electric　field　non-uniformity　factor　at　different　pressures　was　analyzed.　The　applicability　of　streamer　criterion　in　low-pressure　air　gap　was　also　analyzed.　The　results　show　that　the　breakdown　voltage　increases　with　increasing　pressure　and　electrode　distance　for　different　electrode　radius.　At　the　same　air　pressure,　the　maximum　breakdown　field　strength　increases　with　the　decrease　of　the　electrode　radius.　When　the　electrode　radius　r　≥　1.0　cm,　the　streamer　criterion　can　accurately　calculate　the　breakdown　voltage　of　short　air　gaps　at　low　pressures.　　©　2024　IEEE.