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Surface potential decay measurement is a widely used tool to test the electrical properties of insulation materials. However, physical mechanism of the surface potential decay is still poorly understood. In this paper, the effect of corona charging time has been investigated. It has been found that as charging time gets longer initially the surface potential decays faster which is consistent with the existing observations. However, after the charging time reaches a certain length (changing with charging voltage), the surface potential decay becomes slower as the charging time increases. This type of behaviour has not been reported in literature. Further study on this phenomenon using the Pulsed Electro-acoustic (PEA) method which quantifies charge distribution and dynamics inside the sample validated the observation. Finally, the surface potential decay curves were analysed by fitting a double exponential decay equation and simulated by a bipolar charge transport model.