Based on the multimode interference theory and the sensing characteristics of long-period fiber grating (LPFG), a fiber-optic sensor composed of a single mode fiber (SMF)-multimode fiber (MMF)-SMF (SMS) structure and a LPFG is proposed for simultaneous measurement of temperature and refractive index. The experimental results show that the interference spectrum of SMS structure and LPFG have different response sensitivities to temperature and refraction index, and the temperature sensitivity coefficients are 0.017 nm/℃ and 0.060 nm/℃, respectively. The SMS structure is insensitive to refractive index while the refractive index sensitivity coefficient of LPFG is -35.60 nm/RIU. Using the sensitivity matrix, simultaneous measurement of temperature and refractive index is realized. The maximum measurement errors of temperature and refraction index are ±0.59 ℃ and ±0.0013, respectively. With the advantages of high sensitivity, simple structure, immunity to electromagnetic interferences and good linearity, the structure has a excellent application prospect in biochemistry field.