Abstract: To study the influence of air pressure to the discharge of air gap in severe non-uiform electric field, in a metal tank, whose air pressure is adjustable, the rod-plate gap model discharge characteristics of the whole process from corona onset to the breakdown under different rod-plate gap distances, air pressure and voltages were studied with ultraviolet imager. Studies shows that when the air pressure is no more than standard atmospheric pressure, the corona onset voltage of 10cm, 15cm and 20cm gaps and breakdown voltage of 15cm and 20cm gaps approximately meet linear relationship to the air pressure, but the breakdown voltage of 10cm and 4.6cm gaps appear nonlinear properties, and at the same rod-plate gap distance, discharge ionizing light emitting region decreases with the pressure increasing, and approximately meet the relationship of power function, the indexes are in range of -3.8, -2.7, Finally, the discharge in Yunnan (high altitude) and Baoding (plains) region with a same rod-plate gap discharge were analyzed, and the measured results are consistent with experimental results.