This paper deals with the acoustic behaviour and all the possible underwater sound signals of Finless Black Porpoise, which is common in China, under semi-natural condition.After analysing and processing the underwater sound signals in laboratory, three kinds of echo location pulse are found in a specific environment: (1 )low frequency modulated pulse (1-2kHz); (2)high frequency modulated pulse (6-8 kHz); (3)plosive modulated pulse.The paper also presents more than ten different types of continous sound signals and their frequency spectrums. Comprehensive analyses of the relationship between the sound signals and the field behaviour data of Finless Black Porpoise prove that its echo location ability depends mainly on the three kinds of sound pulse mentioned above, while continuous sound is secondery And communication (or language ) between the Finless Black Porpoises is performed by means of several kinds of continous sound signal with harmonic feature.