The invention discloses a monitoring device for a settlement cavity value of a medium-low-speed magnetic suspension low track bearing-rail beam and a roadbed. The monitoring device comprises an inductive strainometer and an inductive sensor testing instrument. The inductive strainometer comprises a ground anchor threaded steel bar set, a lower flange plate, an induction coil, an upper flange plate, an anchor plate and a magnet. The roadbed can drive the induction coil to settle synchronously when settlement happens. A bearing-rail beam bottom slab can drive the magnet to settle synchronously when settlement happens. When the settlement generated by the roadbed is different from the settlement generated by the bearing-rail beam bottom slab, the magnet and the induction coil cut each other to generate currents, and therefore the settlement difference value of the bearing-rail beam bottom slab and the roadbed can be obtained through the inductive sensor testing instrument. According to the monitoring device, data reading is conducted outside a closed loop of the roadbed and prevented from being influenced by the closed loop, the large transportation load of a train, and track panel space limitation of the top of the bearing-rail beam, and the settlement difference value between the bearing-beam rail bottom slab and the roadbed can be observed and measured at any time.