Based on the theory of mode interference and the sensing characteristics of fiber Bragg grating, a combined sensor composed of a single modefiber (SMF)-few mode fiber Bragg grating (FBG)-SMF structure is proposed. The SFS structure interferometer is constructed by fusing a certain length of few mode fiber (FMF)between two SMFs with afiber fusionsplicer, and then FBG is etched on FMF. The transmission spectrum is obtained by the optical spectrum analyzer after the interaction of mode interference and coupling. Firstly, the sensing principle is analyzed. Since the change of environment will cause the effective refractive indexchange of the core mode in FMF, which will cause the wavelength shift of interference spectrum and FBG, the measured parameters can be realized by detecting the wavelength shift of transmission spectrum. Then the effects of FMF length on interference spectrum are simulated. The longer FMF is, the more obvious the interference spectrum is and the smaller the free spectrum range is. In order to observe the transmission spectrum of the combined sensor, the length of FMF is chosen as 110 mm for sensing experiment. FMF can stably transmit four modes with LP01, LP11, LP21 and LP02. By comparing and analyzing the interference and coupling between different modes, it is determined that the interference spectrum is formed by LP01-LP11, and the transmission spectrum of FBG is formed by LP02-LP02, LP11-LP11, LP01-LP02 and LP01-LP01. Finally, the temperature and refractive index sensing experiments are carried out. The results show that the interference spectrum of SFS structure appears obvious blue shift and the transmission spectrum of FBG appears red shift with the increase of temperature. Their temperature response sensitivities are -62.04 and 10.87 pm·℃-1 respectively with good linearity. When the cladding of FMF is corroded to 22 μm, there is no obvious shift phenomenon in the transmission spectrum within therange of 1.366~1.455 andthe maximum sensitivity is only 3.933 nm·RIU-1. The interference peak and transmission peak are used to monitor the environment changes at the same time, which improves the detection accuracy and reduces the accidental errors. The structure has the advantages of novel structure, high sensitivity, easy preparation, and the four resonance peaks of FBG have strong sensing consistency, which makes the sensing more flexible and convenient.