Due　to　the　limitation　of　voltage　capability　and　current　capability　of　power　devices,　it　is　very　difficult　to　handle　nonlinear　loads　for　the　traditional　active　power　filter　(APF)　with　two-level　inverter　in　high　voltage　high　power　grid.　Three-level　NPC　inverter　has　been　put　into　practical　use　for　years　especially　in　high　voltage　high　power　grid.　This　paper　researches　the　APF　with　three-level　inverter　and　presents　a　close-loop　digital　control　algorithm　of　DC　voltage　for　the　APF.　In　dq　two-phase　rotating　coordinate,　this　paper　divides　the　control　system　of　DC　voltage　into　the　outer-loop　voltage　controller　and　the　inner-loop　current　controller.　In　the　outer-loop　controller,　the　mean　value　of　DC　voltage　is　detected　by　the　adaptive　filter　and　regulated　by　the　PI　controller　using　active　command　current.　In　the　current-loop　controller,　this　paper　researches　the　issues　of　neutral-point　voltage　control　from　the　space　vector　PWM　(SVPWM)　point　of　view　in　detail.　Then　this　paper　proposes　a　simple　neutral-point　voltage　control　method　that　requires　the　knowledge　of　the　current　direction　in　each　phase　and　the　neutral-point　voltage　ripple＇s　direction.　Based　on　that　information,　the　small　vectors　that　will　move　the　neutral-point　voltage　in　the　direction　opposite　from　the　direction　of　unbalance　can　be　selected.　The　experiment　results　illustrate　that　the　performance　of　the　proposed　approach　is　satisfactory.