The invention relates to an underwater laser time service system and method based on Beidou. The underwater laser time service system based on Beidou comprises an unmanned aerial vehicle mobile base station which is used for receiving navigation signals and time service information t transmitted by Beidou satellites, carrying out navigation according to the navigation information, moving to a position above an underwater time service receiving device, obtaining precise time according to time service time t, generates a laser signal comprising the precise time and transmitting the laser signal to the underwater time service receiving device; and the underwater time service receiving device which is used for receiving the laser signal, carrying out analysis to obtain standard time of the unmanned aerial vehicle mobile base station, detecting depth information h<2> of the position at which the underwater time service receiving device is located, carrying out data processing on height information h<1>, laser transmitting angle information theta, the depth information h<2> and a water refractive index n through combination of the refractive index n of the water area, thereby generating transmission delay tau, and obtaining the time service time T through combination of the time service information t and the transmission delay tau. Compared with the prior art, the system and the method have the advantages that the transmission delay resulting from signal transmission can be reduced, the anti-interference capability is improved, and the underwater time service precision is improved.