An adapter for an induced draft fan includes an installation section; a transition section; and an exit section. The installation section and the exit section are disposed on two ends of the transition section, respectively. The installation section includes an inner ring, an outer ring, a connection plate, and a water storage cavity. The outer ring includes a first end and a second end; the first end of the outer ring is connected to the transition section; the second end of the outer ring is connected to a first end of the inner ring via the connection plate. The inner ring, the connection plate, and the outer ring are connected one by one to form the water storage cavity. The outer ring includes at least one drain hole communicating with the water storage cavity; and a drain pipe is disposed on the outer ring.