The　non-characteristic　harmonic　analysis　of　±800　kV　UHVDC(Ultra　High　Voltage　Direct　Current)　transmission　system　provides　reference　for　the　optimal　design　of　filters　at　AC　and　DC　sides　of　converter　station　and　for　the　harmonic　mitigation　and　electricity　quality　improvement.　The　mathematic　model　of　converter　is　built　by　the　unified　non　-　characteristic　harmonic　load　flow　algorithm,　which　considers　three　asymmetry　factors　and　the　zero　sequence　current　flowing　through　the　transformer　groups　of　Y/▵　connection.　The　calculated　results　of　the　unified　fundamental　and　characteristic　harmonic　load　flow　algorithm　are　adopted　as　the　initial　values　of　the　unified　non-characteristic　harmonic　load　flow　algorithm.　The　corresponding　mathematic　model　of　the　converters　together　with　the　whole　system　equation　are　set　up　and　solved　with　the　Gauss-Seidel　iteration　algorithm.　The　non-characteristic　harmonics　of　a　±800　kV　UHVDC　transmission　system　are　analyzed　and　calculated　with　the　proposed　unified　non-characteristic　harmonic　load　flow　algorithm.