A magnetic tag sensor includes a cylinder and threaded pipe embedded therein and simulates a magnetic dipole; two threaded pipe wiring interfaces being connected to first and second cables, running through a cylinder upper cross-section outer wall and extending out of the cylinder; the cylinder is sleeved on a guide rail and at a junction between a riverbed and water; an guide rail end inserts into the riverbed, a water sealing box is mounted on a top of the guide rail, a power supply module, a relay and a load arranged inside the water sealing box, the first cable connected to a positive pole of the power supply module, and the second cable connected to a negative pole through the relay and load connected in series; and the threaded pipe in the wall of the cylinder moves up and down with the riverbed to generate a magnetic field signal.