To investigate the bulk charging behavior of ethylene propylene rubber subjected to surface charge accumulation, surface potential decay (SPD) measurement and electro-acoustic method were employed, after the sample was treated by a corona discharge for different time durations. The space charge behavior and trap distribution were obtained. The results show that the surface potential decay (SPD) property correlated with charging time and surface charge polarity. The decay rate of surface potential correlates with charging time, where SPD rate decreases with the increase of charging time. The accumulated surface charges can be injected into the bulk state, and the bulk charging property has a strong correlation with corona treatment time duration. The long-time corona treated sample has a lower initial SPD value. Surface trap distribution influences the accumulation and injection characteristics of the carriers. A plurality of charges will be injected into the material and then trapped as space charges in the interior material. As a result, the trapped space charge can affect surface potential significantly, especially for samples with longer corona treatment duration.