Lithium-ion　batteries　are　widely　used　in　various　fields,　such　as　electronic　products,　energy　storage　and　transportation,　etc.　Accurate　estimation　of　the　state　of　charge　(SOC)　of　lithium-ion　batteries　is　essential　for　maintaining　the　safe　operation　of　batteries.　In　this　work,　considering　the　multilayer　porous　properties　of　lithium-ion　batteries,　a　theoretical　model　of　ultrasonic　reflection　and　transmission　characteristics　is　established,　by　combining　the　state　vector　and　Legendre　polynomials.　Spectrums　of　reflection　and　transmission　coefficients　of　the　lithium-ion　battery　under　charged　and/or　discharged　conditions　are　obtained,　which　are　calculated　theoretically　and　simulated　by　finite　element　analysis.　Meanwhile,　in　order　to　extract　the　shift　properties　of　the　ultrasonic　reflection/transmission　spectrums　versus　SOC,　the　sensitive　points　representing　the　local　extreme　values　in　the　frequency　and　angular　spectrums　during　charging　or　discharging　were　curve　fitted.　It　is　found　that　with　the　increase　of　SOC,　the　local　extreme　value　points　in　frequency　spectrums　intend　to　shift　to　the　high　frequency　side,　and　angular　spectrums　will　shift　to　the　small　angle　side.　Moreover,　an　ultrasonic　reflection　experimental　system　was　built　to　measure　the　reflection　coefficient　spectrum　of　two　kinds　of　batteries　with　different　specifications.　By　comparing　with　the　theoretical　calculation　results,　it　is　found　that　the　experimental　and　theoretical　results　are　in　good　agreement.　And　the　results　also　show　that　the　SOC　is　closely　related　to　the　ultrasonic　reflection　characteristics.　Therefore,　this　research　provides　a　new　way　to　evaluation　the　SOC　of　the　lithium-ion　battery.　©　2023　Elsevier　Ltd