To　investigate　the　influence　of　signal　choice　on　flaw　detection,　detection　is　done　on　single　flaw　by　using　4　tone　bursts　with　different　cycles　at　120　kHz　in　a　7.12　m　long　pipe.　The　tone　burst　of　20　cycles　is　suitable　for　flaw　detection　in　the　pipe　by　comparing　and　analyzing　corresponding　experimental　results.　When　its　cycle　number　becomes　smaller,　the　tone　burst　is　unsuitable　to　detect　flaws　because　the　waveform　of　excited　L(0,　2)　mode　becomes　narrower,　and　its　frequency　spectrums　become　wider,　signal-to-noise　ratio　(SNR)　of　flaw　signals　become　smaller,　and　the　dispersion　of　converted　modes,　such　as　flexural　modes,　becomes　more　serious　when　the　incident　mode　reflects　from　the　flaw.　When　its　cycle　number　gradually　increases　to　a　certain　value,　frequency　spectrum　of　excited　L(0,　2)　mode　becomes　narrower,　its　dispersion　becomes　more　inconspicuous,　and　the　SNR　of　flaw　signals　become　larger,　thus,　the　tone　burst　with　this　number　of　cycles　is　suitable　for　flaw　detection.　However,　the　tone　burst　becomes　improper　when　its　cycle　number　increases　largely.　Because　flaw　echoes　are　inclined　to　superpose　mutually　when　durations　become　longer.