The utility model provides an automatic monitoring device based on a grating fiber, and a steel cage and a pile foundation with the same. The automatic monitoring device comprises a reinforcing steelbar, a grating fiber strain sensor and a fiber wire, a first groove is formed in the whole steel bar body; and a plurality of grating optical fiber strain sensors which are connected with the opticalfiber conducting wires for transmitting data and are used for detecting the change of the internal force of the reinforcing steel bar are arranged in the first groove. According to the utility model,the effect of dynamically monitoring the internal force in real time in the process of loading and operating reinforcing steel bars, steel cages and pile foundations is realized.