The　high-order　and　complex　small-signal　equivalent　circuit　of　the　resonant　converter　prevents　it　from　providing　plentiful　physical　insights.　This　paper　investigates　the　order　reduction　and　approximation　of　the　equivalent　circuit　of　the　series　resonant　converter　(SRC).　It　is　shown　that　there　is　a　close　interaction　between　the　resonant　inductor　branch　and　the　resonant　capacitor　branch　that　must　be　taken　care　of　when　conducting　approximation.　Using　this　methodology,　a　concise　third-order　equivalent　circuit　of　the　SRC　is　proposed.　It　corrects　the　phase　deviation　problem　of　the　existing　reduced-order　model　when　the　switching　frequency　is　close　to　the　resonant　frequency,　thus　is　more　accurate　in　stability　judgment　and　the　closed-loop　design.　Finally,　the　simulation　results　verify　the　validity　of　the　equivalent　circuit.　©　2021　IEEE.