Aiming　at　difficult　problems　to　solve　the　dispersion　characteristics　of　ultrasonic　guided　wave　in　complex　waveguide　structures,　such　as,　rails,　here　the　vibration　modal　analysis　method　was　used　to　extract　the　propagation　characteristics　of　ultrasonic　guided　wave.　Based　on　the　principle　that　vibration　characteristic　frequencies　and　wave　solutions　can　convert　each　other,　the　solutions　of　characteristic　frequencies　obtained　with　vibration　modal　analysis　were　converted　into　wave　solutions,　and　then　the　dispersion　characteristics　of　rails　were　extracted.　Deformation　information　was　gained　from　vibration　analysis　of　a　structure,　and　with　it　the　wave　structure　of　ultrasonic　guided　wave　was　calculated.　The　dispersion　information　and　wave　structure　information　of　lower　order　modes　of　ultrasonic　guided　wave　were　analyzed,　the　modal　type　of　ultrasonic　guided　wave　and　its　frequency　range　were　selected,　all　of　them　provided　a　theoretical　support　for　designing　special　ultrasonic　guided　wave　sensors　and　measuring　tests.