Propagation　characteristics　of　longitudinal　ultrasonic　guided　waves　of　steel　rod　embedded　in　infinite　medium　are　studied　in　theory.　Due　to　the　characteristic　of　dispersion　and　multi-mode　of　rod,　a　longitudinal　mode　whose　attenuation　is　smallest　in　all　modes　and　its　energy　velocity　which　is　the　fastest　at　special　frequency　can　be　excited.　This　high　frequency　loss　attenuation　mode　can　propagate　longer　distance　and　be　received　firstly,　which　indicates　that　this　mode　can　be　used　to　the　inspection　of　rod　embedded　in　infinite　medium.　At　the　same　time,　the　experiments　about　diameter　22　mm,　1　m　length　rod　embedded　in　grout　detected　using　ultrasonic　guided　wave　are　carried　out,　and　time-frequency　analysis　is　also　performed　for　the　results　of　experiments.　Experiment　results　are　consistent　with　those　of　theory.