Sound plays an important role in dugong s life activities. In order to study the vocal types and characteristics of dugongs，underwater vocalizations of dugongs in waters near Libong Island, Thailand, were recorded from March 20 to March 28，2019. Acoustic signals of dugongs in Thai waters were classified in detail and the characteristics of each type of acoustic signal were counted in this paper. The acoustic signals of wild dugongs in waters near Libong Island could be divided into four types: chirp, trill, bark, and whistle. We chose signals with a high signal-to-noise ratio, clear contour, and no overlap for each type of acoustic signal of dugongs for feature statistics. Chirps were short, narrow-band frequency modulation signals with a time duration of 0.09 s. The fundamental frequency of chirps was between 0.99-12.84 kHz and the harmonics were 1一5. Trills were frequency modulation signals with a longer duration time generally more than 0.90 s; the fundamental frequency was between 0.97- 9.89 kHz. and there were 1- 5 harmonics. The average time duration of barks was about 0.20 s and the bandwidth ranged from 0.20 to 4.00kHz. Whistles were defined as transitional signals between chirps and trills. The time duration of whistles was between 0.05-0.89 s with 1- 5 harmonics and the fundamental frequency was between 1.11 - 7. 69kHz. The proportions of these four types of acoustic signals were different. Chirps accounted for the most (more than 86%) and barks were the least (less than 1%). The proportions of trills and whistles were about 7% and 6% respectively. The research on the types and characteristics of the acoustic signals of dugongs may be helpful to analyze the acoustic behavior of dugongs, and it will have important guiding significance for acoustic observation of wild dugongs.