The invention discloses a novel tooth root bending stress online monitoring device and a novel tooth root bending stress online monitoring method based on a fiber grating. The novel tooth root bending stress online monitoring device comprises a signal detecting unit and a signal processing unit. The signal detecting unit comprises a measurement grating and a transmission optical fiber which are arranged on the side surface of a gear. The transmission optical fiber is closely adhered on a position which is next to a tooth tip on the side surface of the gear. The measurement grating is arranged on the transmission optical fiber between two adjacent gear teeth. The signal processing unit comprises an optical fiber slip ring, a fiber grating demodulator and a computer. The transmission optical fiber penetrates through the optical fiber slip ring and is connected with the fiber grating demodulator. The fiber grating demodulator is connected with the computer through a transmission netting twine. The measurement grating acquires the signal of the gear in real time. After processing by the fiber grating demodulator, the signal of the gear is converted to a tooth root bending stress, and furthermore a processing result is transmitted to the computer for realizing real-time display. The novel tooth root bending stress online monitoring device and the novel tooth root bending stress online monitoring method can realize gear transmission bending stress monitoring in various working conditions, and furthermore have advantages of simple structure, convenient operation, accurate measurement and easy realization.