Abstract: The problem of sonar signal waveform design to detect a high speed small target in an underwater en- vironment is discussed. This paper point out that the suitable signal waveform is time-frequency hop signal with multi-frequency. To get precise target parameter estimation, the signal should have high distinguish performance in both time and frequency fields. The result of signal analysis show that the hop signal with frequency serial coding as Costas array has sharp ambiguity characteristic, so it can be used in an active sonar system to detect a high speed small target.