The　state　of　charge　(SOC)　of　lithium-ion　batteries　can　reflect　the　usage　of　batteries.　When　the　battery　has　problems　or　aging,　the　amount　of　charge　must　be　affected.　Knowing　the　change　in　battery　SOC　in　advance　by　using　ultrasonic　detection　will　reduce　the　harm　caused　by　aging.　During　the　charge,　the　elastic　modulus　and　density　of　the　internal　electrode　vary.　This　will　affect　the　propagation　of　ultrasonic　waves　inside　the　battery,　especially　for　the　reflection　coefficient　and　transmission　coefficient.　In　this　research,　the　Legendre　orthogonal　polynomial　method　is　adopted　to　solve　this　problem.　When　SOC　varies,　the　reflection　and　transmission　coefficient　curves　show　regular　shift　while　the　amplitude　changes　accordingly.　It　proves　that　both　the　coefficients　are　capable　of　estimating　battery　SOC.　A　finite　element　model　of　lithium-ion　battery　was　established　in　the　COMSOL　simulation　software.　Both　the　reflection　and　transmission　coefficients　obtained　were　consistent　with　the　theoretical　prediction,　which　further　verified　the　feasibility　of　the　method.　©　2019　IEEE.