Line-filtering　electrochemical　capacitors　(LFECs)　are　demonstrating　advantages　in　line　filtering　over　traditional　electrolytic　capacitors.　However,　they　can　only　function　at　no-load　or　low-power　conditions　due　to　the　limited　high-frequency　capacitance　resulting　from　the　excessive　ionic　resistance,　despite　much　progress　in　electrode　materials.　Here,　we　show　separators　dominate　both　ion　migration　and　capacitance　in　LFECs.　A　3　μm-thick　thread-anchor　structured　separator　is　developed,　featuring　both　accelerated　ionic　transport　and　reliability,　leading　to　a　low　ionic　resistance　of　25　mΩ　cm2.　With　a　phase　angle　of　−80°　at　120　Hz,　the　assembled　device　has　an　areal　capacitance　of　6.6　mF　cm−2.　Furthermore,　stack　integration　in　parallel　breaks　the　trade-off　between　capacitance　and　frequency　response,　boosting　the　areal　capacitance　by　two　orders　of　magnitude　without　decay　of　frequency　characteristics.　The　On-board　field　test　demonstrates　that　voltage　ripples　are　steadily　suppressed　below　5%　even　for　practical　high-power　line　filtering　with　a　load　power　density　of　2.5　W　cm−2,　three　orders　of　magnitude　higher　than　previous　instances.　This　work　opens　up　a　perspective　of　separator　engineering　for　the　development　of　high-performance　line-filtering　electrochemical　capacitors　and　promotes　their　applications　in　practical　high-power　scenarios.　©　The　Author(s)　2025.