Longitudinal　ultrasonic　guide　wave　dispersion　equation　of　inviscid　liquid-filled　pipe　is　analyzed　theoretically.　Propagation　characteristics　of　longitudinal　modes　in　water-filled　steel　pipe　and　water　cylinder　with　rigid　boundary　are　studied.　Except　α　mode,　other　modes　in　wall-filled　steel　pipe　are　the　coupling　of　the　modes　of　steel　pipe　in　vacuum　and　of　water　cylinder　with　rigid　boundary.　To　the　same　mode　in　water-filled　steel　pipe,　its　propagation　characteristics　vary　greatly　with　frequency　range.　Furthermore,　longitudinal　modes　are　excited　and　received　in　steel　pipes　filled　with　and　without　water　experimentally.　Results　show　that　propagation　characteristics,　such　as　mode　branching　and　group　velocity　decrease,　occur　in　water-filled　steel　pipe.　When　frequency　band　of　longitudinal　mode　in　water-filled　steel　pipe　is　in　the　range　of　one　unperturbed　L(0,　2)　mode　branching　of　steel　pipe　in　vacuum,　the　mode　has　the　ability　of　long　distance　propagation,　and　the　signal　shape　and　amplitude　are　retained　as　it　travels　long　distances　for　its　low　dispersion.　As　a　result,　the　kind　of　longitudinal　mode　is　suitable　to　defect　detection　of　long　distance　water-　filled　pipe.