The invention relates to a method for realizing simultaneous sensing of temperature, strain and refractive index through a single fiber bragg grating (FBG). A localized FBG is written in a few-mode fiber by using femtosecond laser. The FBG transmitted spectrum is provided with multiple bragg resonance peaks and multiple cladding mode resonance peaks. All the bragg resonance peaks have different responses to the temperature and the strain and are not sensitive to the external refractive index. All the cladding mode resonance peaks have different responses to the external refractive index and the strain and are not sensitive to the temperature. The amount of movement of two bragg resonance peaks selected in the FBG transmitted spectrum and the normalized area ( the specific value of the area formed by enclosing of the upper and lower envelop of the cladding mode resonance peaks to the initial area) of the cladding mode resonance peaks are measured so as to realize simultaneous sensing of the temperature, the strain and the refractive index through the single FBG. The beneficial effects of the method are that the method has high sensing sensitivity, the signals are easy to demodulate, the sensing head is easy to manufacture and the mechanical strength is high with no requirement for special packaging.