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With the broad application of silicone rubber (SIR) insulator in high-altitude regions, corona degradation is one of the inevitable electrical degradation caused by electric stress due to the organic properties of SIR. Although this phenomenon is investigated at atmospheric pressure, there is little investigation at low pressures. It is urgent to systematically investigate effects of low pressure on the corona degradation for the reliable application. Experiments were carried out in a decompression chamber and the ambient pressure was reduced by a rotary pump from the atmospheric pressure to 40 kPa. The specimens were exposed to corona discharges at low pressures for different lapse time. The surface static contact angles on the insulator were measured to reveal the hydrophobicity changes. The currents during corona discharges at different low pressures were measured to confirm the corona degradation with variation of the pressures. The results obtained showed that the intensity of corona discharge increased with reducing the ambient pressure, which accelerated the process of corona degradation.