An adapter assembly for an inflating device has a case body and a warning unit. The case body has a first chamber and a second chamber. The first chamber and the second chamber are formed in the case body and communicate with each other. The warning unit is mounted in the second chamber and has a selection module, multiple adjusting modules, a sounding body, and a cover body. The selection module is mounted in the second chamber and has multiple accommodating portions formed in the selection module. One of the accommodating portions communicates with the second chamber. The adjusting modules are respectively mounted in and communicate with the accommodating portions. The sounding body is mounted on the selection module and communicates with the accommodating portions. The cover body is mounted on the selection module, and has a least one sound hole communicating with the sounding body.