The invention relates to the technical field of immersed tunnels, in particular to an immersed tube semi-rigid segment joint waterproof system and a watertight leakage detecting method thereof. The immersed tube semi-rigid segment joint waterproof system comprises a first waterproof belt and a second waterproof belt which are installed between two adjacent segments, the first waterproof belt and the second waterproof belt are each of a closed annular structure distributed in the circumferential direction of an immersed tube segment, a gap is reserved between the first waterproof belt and the second waterproof belt, and a closed annular cavity is formed; the first waterproof belt is located on the inner side of the second waterproof belt, the multiple waterproof belts are arranged between the segments, and therefore the effective segment joint waterproof system is formed, and the problem of water leakage at segment joints is solved; and meanwhile, by means of the watertight leakage detecting method of the immersed tube semi-rigid segment joint waterproof system, simulation watertight testing is carried out on the waterproof system, it is guaranteed that the waterproof system has nofaults in the using process, leakage can be avoided, and the reliable scientific guarantees are provided for construction quality.