For　the　first　harmonic　approximation　(FHA),　it　is　difficult　to　obtain　accurate　gain　results　when　it　deviates　from　the　resonant　frequency　point,　and　it　is　not　suitable　for　the　application　of　wide　gain　range.　Based　on　this　issue,　a　new　time　domain　analysis　(TDA)　method　is　proposed　to　derive　more　accurate　gain　expressions　of　CLLC　resonant　converters,　so　as　to　guide　parameter　optimization　design.　First,　the　deficiency　of　the　FHA　is　analyzed.　Then,　the　CLLC　converters　in　different　operating　modes　are　modeled　by　TDA,　and　a　more　accurate　gain　expression　is　derived.　Under　the　premise　of　satisfying　soft　switching,　the　inductance　ratio　k　and　the　quality　factor　Q　are　optimized　by　the　derived　precise　gain　formula.　The　gain　formula　obtained　by　this　method　has　high　precision　and　a　simple　parameter　design　process.　Finally,　based　on　the　parameters　of　the　optimized　design,　a　1　kW　experimental　prototype　is　built　that　satisfies　the　wide　gain　range　output.　The　simulation　and　experimental　results　both　verify　the　accuracy　of　TDA　and　the　feasibility　of　the　parameter　design　method.　©　2023　Power　System　Protection　and　Control　Press.　All　rights　reserved.