The invention discloses an underwater waterstop belt leakage detecting method after installing an immersed pipe tunnel. The method comprises the following steps that (1) a water inlet hole and a water outlet hole which communicate with a sealed cavity at a connecting head are formed in the inner wall of a immersed pipe; (2) water injecting and air exhausting are conducted on the sealed cavity, after air is exhausted, an air exhausting hole is closed, water is continuously injected until water pressure reaches design demands, and water injecting is stopped; (3) whether the water pressure in the sealed cavity is descended or not within a period of time is continuously monitored, and stopwater belt situations are checked; (4) if the water pressure in the sealed cavity is gradually descended, leakage points are found and processed, and the step 2 and 3 are repeated until the water pressure keeps unchanged; and (5) water in the sealed cavity is all exhausted. By means of the underwater waterstop belt leakage detecting method after installing the immersed pipe tunnel, after the off-sea immersed pipe is installed, the effect of the stopwater belt at the connecting head can be checked, it is ensured that water tightness of the pipe joint connecting head is good, loss caused by water leakage of the connecting head of the immersed pipe is avoided, and the underwater waterstop belt leakage detecting method has the very important guiding significance for design and construction of similar immersed pipe tunnel engineering.