The invention discloses a double communication system underwater acoustic Modem which integrates a spread spectrum communication system and an SC-FDMA (single carrier frequency division multiple access) communication system respectively applicable to low-speed error-free and high-speed low-error underwater acoustic communication environments. At a transmitting end, the underwater acoustic Modem can transmit spread spectrum communication signals or SC-FDMA communication signals according to concrete commands given by an upper control module. At a receiving end, the underwater acoustic Modem has an automatic communication system authentication function, and can automatically authenticate the signal receiving communication systems under the condition of lacking prior information of the signal receiving communication systems to perform effective communication. The underwater acoustic Modem has a quite high engineering application value.