Polypropylene　(PP)　is　widely　used　in　power　capacitors　and　potentially　used　in　HVDC　cables　for　its　excellent　electrical　properties.　The　space　charge　injection　and　accumulation　characteristics　under　DC　voltage　are　important　factors　for　electrical　breakdown.　In　order　to　study　the　relationship　between　electrical　breakdown　and　charge　transport,　PP/Al2O3　nanocomposites　with　weight　percent　of　1%,　7%　and　15%　were　prepared　to　conduct　polarization,　thermal　stimulation　depolarization　current,　high　field　conductance,　positron　annihilation　lifetime　spectra　and　DC　breakdown　experiments.　The　experimental　results　show　that　the　DC　breakdown　strength　of　the　1%　nanocomposite　is　improved　compared　to　that　of　pure　PP,　which　corresponds　to　the　deeper　traps.　While　for　7%　and　15%　nanocomposites,　the　breakdown　strength　and　trap　energy　are　both　decreased.　Free　volume　of　the　composites　does　not　significantly　change.　Based　on　the　bipolar　charge　transport　model,　the　dynamic　evolutions　of　space　charge　and　electric　field　varied　with　trap　energy　and　boost　voltage　time　were　numerical　simulated.　The　simulated　results　show　that　both　injected　charges　and　electric　field　distortion　decrease　with　deeper　trap.　The　charge　injection　depth　is　shallower　at　larger　trap　energy.　As　the　time　increases,　the　charge　injection　and　electric　field　distortion　are　more　obvious　after　the　electric　field　reaches　a　certain　threshold.　The　accumulation　of　homo　space　charge　formed　by　trapped　charge　carriers　induced　by　deep　traps　and　the　inhibition　of　carrier　mobility　contribute　to　the　improvement　of　DC　breakdown　performance.　©　2019　Chin.　Soc.　for　Elec.　Eng.