Simultaneous measurement of refractive index and temperature based on an optical fiber cavity sensor was demonstrated, which is a reflective and multi-beam interference based sensor. The sensor head is made up of a normal single mode fiber, a graded-index multimode fiber and an air gap. The air gap just locates between the single mode fiber and the graded-index multimode fiber. The lengths of the graded-index multimode fiber and the air gap are 538.1 μm and 40.8 μm, respectively. The fringe contrast and the wavelength of the interference, due to multiple beam interferences in the sensor head, vary with the RI and temperature of the solution, respectively. Hence, the simultaneous measurements of refractive index and temperature, by monitoring the movement of the fringe contrast and the wavelength of the interference, were achieved. Experimental results show that the proposed optical fiber cavity sensor presents a RI sensitivity of 57.24 dB/RIU in the measurement 1.341 5~1.432 0RIU and a temperature sensitivity of 12.3 pm/℃ in the range of 30 ℃~70 ℃. The minimum temperature that can be detected is 1.2℃，the minimum resolution of RI that can be detected is 3.4×10－4. This optical fiber cavity sensor can also be applied in other sensing fields and has good prospects.