The　development　of　Extra/Ultra　high　voltage　(EHV/UHV)　transmission　requires　high-performance　insulating　dielectrics.　Dielectric　properties　of　cross-linked　polyethylene　(XLPE)　used　in　high-voltage　insulated　cables　are　key　to　ensuring　the　stability　of　EHV/UHV　transmission　line.　Acetophenone　is　one　of　the　main　byproducts　produced　in　XLPE　after　crosslinking　with　dicumyl　peroxide　(DCP).　This　paper　intends　to　quantitatively　investigate　the　effect　of　acetophenone　on　the　dielectric　properties　of　polyethylene.　The　low-density　polyethylene　(LDPE)/acetophenone　composites　with　different　contents　of　acetophenone　are　prepared　by　soaking　pure　LDPE　samples　in　acetophenone　for　various　times.　Gas　chromatography　(GC)　is　employed　to　test　the　contents　of　acetophenone.　The　dielectric　constant,　dielectric　loss　and　AC　breakdown　strength　at　the　power　frequency　(50　Hz)　of　LDPE　and　LDPE/Acetophenone　composites　are　tested.　To　explore　the　underlying　mechanism,　first　principles　method　and　molecular　dynamics　(MD)　simulation　are　employed　to　calculate　the　dipole　moment,　electron　level　and　electrostatic　potential　(ESP)　distribution　of　materials　in　this　paper.　The　experimental　and　calculation　results　show　that　the　dielectric　constant　of　the　LDPE/Acetophenone　composites　increases　with　the　increase　of　acetophenone　content　because　of　the　newly　established　dipolar　polarization,　which　originates　from　the　introduction　of　polar　acetophenone.　While　the　AC　breakdown　strength　of　the　composites　decreases　obviously　as　the　acetophenone　content　increases,　which　is　attributed　to　the　low　band　gap　and　uneven　ESP　distribution　in　them.　Our　results　demonstrate　that　the　existence　of　acetophenone　can　lead　to　the　severe　degradation　of　dielectric　properties.　It　provides　insight　into　the　quantitative　effect　of　the　acetophenone　on　dielectric　properties　of　LDPE　and　provides　guidance　for　improving　the　dielectric　properties　of　XLPE.　©　2022　IEEE.