The　turn-off　loss　of　the　switches　is　the　main　loss　of　the　full-bridge　CLLC　converter　under　high-frequency　conditions,　and　it　is　a　variable　closely　related　to　the　resonance　parameters　of　the　resonant　tank　and　the　voltage　gain.　First,　the　gain　characteristics　of　the　full-bridge　CLLC　converter　are　analyzed　based　on　the　first　harmonic　approximation　(FHA).　Then,　the　precise　functional　relationships　of　the　switches＇　turn-off　current　and　turn-off　loss　are　derived　by　time　domain　analysis　(TDA).　On　this　basis,　the　constraint　space　of　resonance　parameters　is　reduced　based　on　the　monotonicity　constraints　of　the　gain　curve,　soft　switch　realization　conditions,　overall　efficiency　constraints,　and　turn-off　loss　characteristics　constraints,　and　the　particle　swarm　optimization　(PSO)　algorithm　is　used　to　solve　the　resonant　parameter　under　the　turn-off　loss　optimization.　Finally,　a　simulation　model　was　built　through　optimized　parameters,　and　prototype　experiments　were　carried　out.　Meanwhile,　compared　with　the　traditional　loss　optimization　method,　it　is　proven　that　the　proposed　method　can　make　the　output　voltage　closer　to　the　design　value　and　reduce　the　turn-off　current　and　turn-off　loss　under　the　premise　of　ensuring　soft　switching.