A leakage detecting device for a watertight door of an immersed tunnel comprises a water tank, a test platform and an increasing air pump. The water tank is composed of a bottom panel, a top panel and side panels, one side end of the water tank is open, connecting plates are arranged on the periphery of the opening end and provided with connecting holes, and the increasing air pump is connected with the water tank through a valve. The test platform is composed of a main panel, cross beams, vertical beams, a hand wheel and pull rod system and a hinge. Through holes corresponding to the connecting holes in the connecting plates of the water tank are formed in the periphery of the main panel. The connecting holes in the connecting plates of the water tank and the through holes in the main panel are connected through fastening bolts. The main panel is provided with observation holes. The hinge is arranged on the outer end face of the main panel. Fixing box iron of the hand wheel and pull rod system is installed on the inner end face of the main panel. One end of each pull rod is connected with a handle wheel, and the other end of each pull rod is a free end and penetrates the fixing box iron and the main panel. Due to the fact that the test platform and the water tank are adopted as test tools, operation is easy and convenient, the watertight door can be subjected to leakage detection in a workshop in advance, the construction period of the immersed tunnel can be greatly shortened, and the work efficiency is improved.