A sensing device is configured for placing in an air cushion bed below air bladders thereof to detect an exact state of the bed in use, and includes a first and a second sheet connected together to define an air chamber therebetween. The air bladders and the air chamber are communicable with one another via inflation valves on the first sheet and are synchronously and identically inflated. First and second sensing elements are disposed outside and inside the air chamber, respectively. The first sensing element has deformation and acceleration sensors arranged thereon for monitoring how the bed reacts to a patient lying thereon and the manner in which the bed is being used. The second sensing element includes upper and lower conductive sensing sheets and an electrical connection state therebetween is used to detect whether the air bladders are sufficiently inflated. Thus, the air cushion bed is safer for use.