The　characteristic　equation　of　Lamb　waves　in　a　solid　plate　with　loading　liquid　on　single　side　is　derived.　The　dispersion　curve　is　also　calculated.　A　new　method　monitoring　the　fixed　liquid　level　is　presented　and　implemented,　based　on　the　sensitivity　of　the　characteristic　parameters,　such　as,　attenuation,　of　some　Lamb　wave　modes　to　the　presence　of　liquid.　The　results　show　that　the　presence　of　liquid　has　little　influence　on　the　phase　velocity　dispersion　curve　and　energy　velocity　dispersion　curve,　but　great　influence　on　the　attenuation　dispersion　curve;　the　experimental　results　are　in　good　agreement　with　the　theoretical　calculations　and　imply　that　this　method　is　suitable　for　monitoring　the　fixed　liquid　level　of　large　tanks.