The invention discloses a water draining system of jointing cavities of an immersed tunnel final joint. Two immersed tubes connected with the final joint are each internally provided with a water storage device and a water draining device in advance. An end sealing door, connected with one end of the final joint, of each immersed tube communicates with the corresponding water draining device, andeach water draining device communicates with the corresponding water storage device. By means of each water draining system, the water draining devices arranged in the immersed tubes communicate withthe end sealing doors and the water storage devices, water in jointing cavities at the two ends of the final joint is drained to the water storage devices through the water draining devices communicating with the end sealing doors, the problems that when immersed tubes are laid from the two ends to the middle to build an immersed tunnel, the final immersed tube, namely the final joint is connectedwith the two immersed tubes to be closed at the same time, two jointing cavities are formed, water cannot be directly drained to a sea (rivers) through an existing jointing cavity water draining method, and meanwhile to guarantee the watertightness and the integral structural stability of the immersed tunnel, the immersed tubes cannot be provided with holes for water draining can be solved, and the system is simple in structure and convenient to use and popularize.