The invention discloses a device for monitoring a steel bar yield state in a reinforced concrete bridge structure. The device for monitoring the steel bar yield state in the reinforced concrete bridge structure comprises a bracket, a Dewar flask, a magnetic shielding material layer and a magnetic probe, wherein the magnetic shielding material layer is arranged at the bottom of a liquid storage cavity in the Dewar flask; liquid nitrogen is encapsulated in the Dewar flask; the magnetic probe is arranged below the Dewar flask, and is located in a range of a region covered by the magnetic shielding material layer; the magnetic probe and the Dewar flask are connected into a whole through a bracket; and the magnetic shielding material layer adopts yttrium barium copper oxide. The invention further discloses a method for monitoring steel bar strength and a steel bar yield state in a reinforced concrete bridge structure. The device has the beneficial effects that a novel device capable of monitoring steel bar yield in reinforced concrete is provided; and a novel safe bridge monitoring method is designed according to the monitoring result of the monitoring device, and can be implemented on newly built bridges and existing bridges.