The　assembly　of　high-temperature　electronic　equipment　places　great　demands　on　ultra-high　temperature　multilayer　ceramic　capacitors　(UHT-MLCCs).　However,　the　relatively　low　dielectric　constant　and　inferior　reliability　associated　with　the　protruding　ferroelectric　phase　transition　for　existing　dielectric　materials　have　hindered　the　development　of　UHT-MLCCs.　Here,　these　concerns　have　been　addressed　by　the　strategy　of　composition　modulation　of　different　types　of　polar　nanoregions　(PNRs)　in　bismuth-based　perovskite　relaxor.　A　new　lead-free　dielectric　system　(1-x)(0.8Na(0.5)Bi(0.5)TiO(3)-0.2K(0)(.5)Bi(0.5)TiO(3))-xBi(Mg2/3Nb1/3)O-3　(NBT-KBT-xBMN)　dominated　by　P4bm　PNRs　has　been　built　and　the　corresponding　UHT-MLCCs　with　70Ag/30Pd　inner　electrodes　are　fabricated　by　tape　casting　and　cofiring　processes.　A　record-high　dielectric　constant　(epsilon(r)　=　1800　+/-　15%)　together　with　low　dielectric　loss　(tan　delta　＜　0.025)　is　achieved　over　a　wide　range　of　100　degrees　C-440　degrees　C　for　NBT-KBT-0.2BMN　MLCC,　which　consists　of　nine　dielectric　layers　with　the　single-layer　thickness　of　70　mu　m.　The　excellent　results　suggest　that　NBT-KBT-0.2BMN　MLCC　can　provide　realistic　solutions　for　next-generation　UHT-MLCC　applications　and　the　material　design　strategy　can　be　generalized　for　the　construction　of　more　high　performance　capacitor　dielectrics.