In　this　article,　a　high-efficient　approach　is　presented　for　obtaining　S-parameters　from　coupling　matrix　of　a　microwave　filter　whether　it　is　lossless　or　lossy　(assuming　all　the　resonators　with　the　same　unloaded　Q).　Computing　time　is　usually　large　because　inversion　matrix　needs　to　be　carried　out　once　at　each　frequency　in　traditional　method.　By　making　the　application　of　matrix　inversion　Lemma　and　eigenvalue　decomposition　theory,　the　matrix　inversion　operation　is　replaced　by　one-time　matrix　eigenvalue　decomposition　in　the　whole　process,　no　matter　how　many　frequency　we　need　to　concern.　After　reorganizing　the　equations　of　the　S-parameters,　the　time　complexity　is　decreased　from　O(N3)　to　O(N).　An　example　of　14th　cavity　cross-coupled　filter　is　reported　for　illustrating　the　capabilities　of　the　new　proposed　approach.　Compared　with　the　general　method,　the　time　spending　on　computing　was　shortened　by　70%　with　our　new　approach.　Therefore,　the　proposed　approach　will　be　very　popular　and　suitable　in　the　industry.　©　2018　Wiley　Periodicals,　Inc.