At lower humidity

intense insulation discharge occurred in 500 k V Gangcheng substation. We apply the chemical analysis and electrical test to investigate the result of discharge activities. Chemical simulation shows that the porous structure enlarges the specific surface area to absorb more moisture. Four-coordinated aluminum atom and bridge oxygen atom are able to form hydrogen bonds with water molecular. These two factors lead to strong moisture absorption capacity of aluminum phosphate. The results of artificial pollution test show that the discharge inception voltage of aluminum phosphate contaminant is much lower than that of regular contaminant at the unsaturated humidity. As the salt deposit density（SDD） is 0.20 mg/cm2

the discharge inception voltage of the regular polluted insulator is 17.5 k V at relative humidity of 70%. At the same condition

the discharge inception voltage of polluted insulator with the aluminum phosphate mass fraction of 20% is low to 10.0 k V. The investigation reveals that the aluminum phosphate contaminant leads to the discharge actives in Gangcheng substation. External insulation discharge characteristic is affected by the contaminant moisture absorption. The contaminant with strong moisture absorption capacity can reduce the humidity for insulation discharge.