Abstract: The conductivity, space charge and breakdown strength are the key indicators to evaluate the electrical properties of polypropylene (PP) for HVDC cables. Two isomeric hindered phenolic antioxidants 4,4’-thiobis (6-tert-butyl m-cresol) (AO300) and 4,4’-thiobis (6-tert-butyl o-cresol) (AO736) are selected to investigate the effects of antioxidants on conductivity, space charge and breakdown after polarity reversal properties for PP. The experimental results show that these two antioxidants can introduce deep trap sites into PP, thereby increasing the conductivity activation energy, reducing the space charge accumulation, and improving the DC breakdown strength after polarity reversal. Based on quantum chemistry calculations, the energy band distributions and bond dissociation energy (BDE) of the two antioxidants are obtained. It is found that AO736 can introduce more trap sites than AO300, which capturing high-energy electrons and suppressing the destruction process of PP molecular chain. Besides, the hydrogen-oxygen bond in AO736 has a lower BDE, reflecting a stronger free radical scavenging ability, and the breakdown process is suppressed by preventing the free radical chain reaction According to the analysis, AO736 can significantly improve the space charge and breakdown after polarity reversal properties of PP for HVDC cables.