Abstract: In order to improve the data rate of long-range underwater acoustic communication (UWAC) system working on low frequency band, a type of communication mode is presented, that is Chaotic-FM M-ary mode. The chaotic sequences into UWAC field is introduced. Since the cross-correlation of the quantized chaotic sequences is damaged, Chaotic-FM signal which has many good properties, such as good correlation properties, numerous orthogonal codes, high utilization ratio of spectrum etc are studied. A block diagram of UWAC system based on Chaotic-FM M-ary mode is given out, and one sea test is designed to verify its feasibility and performance. The experimental results are in agreement with theoretical results. The Chaotic-FM M-ary mode is both feasible and reliable. It can improve the data rate of UWAC system.