An adjustable mattress comprises a carrier and a foundation. The carrier has a bed surface, a bottom surface, and a plurality of first flexible members and second flexible members distributed in the carrier. One of the first flexible members and one of the second flexible members are encased in a flexible support unit. The bottom of the carrier is disposed on the foundation. The two bearing points in a stretchable direction of the first flexible member contact the bed surface and the bottom surface respectively, and one end of the second flexible member protrudes from or is concealed in the bottom surface of the carrier. The foundation has a moving plate and a lifting device. The moving plate can be lifted by the lifting device to an active position to support weights from the bed surface with the first flexible members.