A simultaneous measurement system for temperature and refractive index is proposed using a long period fiber grating(LPFG) and a polarization-maintaining fiber(PMF) Sagnac loop,based on which the transmission spectrum of the fiber Sagnac interferometer is modulated by the LPFG.Through measuring the wavelength shifts and the intensity changes of a resonance dip of the system,the wavelength and the intensity response sensitivities to temperature and refractive index are obtained,and the sensitivity coefficient matrix is constructed.The experimental results show that the wavelength of the dip varies with temperature and the intensity of the dip changes with refractive index.The temperature sensitivity of the system is 0.128 57 nm/℃,and the refractive index sensitivity of LPFG is 49.380 32 dB/RIU.The experimental system is small and easy to set up,whose cost is relatively low and it has good application prospect.