Mechanical　properties　such　as　density　and　Young＇s　modulus　of　lithium-ion　battery　electrodes　are　related　to　the　state　of　charge　(SOC).　Characterizing　the　battery　SOC　by　means　of　ultrasonic　non-destructive　testing　can　obtain　the　relationship　between　wave　propagation　information　and　the　SOC.　During　the　battery　charging　process,　the　Young＇s　modulus　and　density　of　the　internal　electrode　material　will　change,　which　will　affect　the　propagation　of　ultrasonic　waves　in　the　battery.　In　this　research,　a　clear　and　more　comprehensive　description　of　the　reflection　characteristics　of　ultrasonic　waves　in　lithium-ion　batteries　have　been　presented.　The　Legendre　orthogonal　polynomial　method　(LOPM)　was　first　introduced　to　solve　the　reflection　coefficients　of　single-　and　multi-cell　lithium-ion　batteries.　The　angular　spectrum　and　frequency　spectrum　of　which　were　regularly　shifted　with　the　SOC,　so　the　SOC　could　be　characterized　accordingly.　The　results　obtained　by　finite　element　simulation　were　highly　consistent　with　the　method　used.　Moreover,　an　ultrasonic　reflection　experiment　system　was　built,　which　the　result　matched　with　theoretical　calculation　in　a　customized　battery,　further　verified　the　feasibility　of　the　method.　©　2022　Elsevier　B.V.