In this paper, a needle-plane electrode system and polypropylene (PP) films were used to study the DC corona charging of polymers. It was found that the charges injected into the PP films sometimes show different polarity to the applied DC voltage, called polarity reversal charging, which may results in invalidation charging. The pulsed electro-acoustic (PEA) space charge measurement showed that charges of different polarities have been injected into the PP films' bulk, simultaneously. From these results, a mechanism of two charging processes of high-field inection and return-stroke injection has been introduced here. We presumed that the return-stroke injection can be boosted with the increase of corona voltages and thus reduces the net charge as well as even change the polarity of charges injected.