The invention discloses an underwater communication system based on continuous variable quantum key distribution and a realization method thereof, belonging to the technical field of free space quantum communication. The optical signal is outputted to the receiving end through the cooperation of a first laser, a first electro-optic intensity modulator, a signal source, a first beam splitter, a second electro-optic intensity modulator, a first electro-optic phase modulator and a tunable laser attenuator at the transmitting end; The second laser generates local oscillation light, which is modulated by the second electro-optical phase modulator, the local oscillation light and the optical signal outputted from the transmitting end enter the homodyne detector for detection, and finally the security key is established within the effective distance. The invention applies the continuous variable quantum key distribution technology to the underwater optical communication system, the optical signal is not easy to be stolen by a third party, and the safe underwater communication system is realized. Heterodyne detection is used to filter the ambient stray light, which also achieves good detection effect in the daytime and the environment with background light.