It　is　multimode　Lamb　waves　that　compose　the　acousto-ultrasonic　(AU)　signals　when　the　thin　plate　is　tested　by　AU　method.　Because　of　the　complication　of　Lamb　waves,　AU　method　avoids　the　further　analysis　for　signal　components,　and　constructs　so-called　stress　wave　factor　instead　of　measuring　energy　dissipation.　It　is　very　hard　to　recognize　the　signals　with　both　multiple　modes　and　dispersion.　Time-frequency　analysis　is　used　to　separate　the　AU　signals　and　a　single　measurement　is　only　required.　By　combining　the　pseudo　Wigner-Ville　distribution　with　reassignment　technique　and　feature　extraction,　portions　of　dispersion　curves　of　Lamb　waves　can　be　obtained.