We propose a high sensitivity fiber optical temperature sensor with simple structure. The sensor is composed of a multimode fiber (MMF) and a short single mode fiber (SMF) whose end is coated with silver film as a reflective mirror. Due to the mismatch between the MMF core and the SMF core, part of guided mode is coupled with the cladding modes of downstream SMF cladding. The mode index difference between the fiber core and cladding is attributed to the phase difference, resulting in interference. When ambient temperature increases, the interference spectrum presents red-drift because of the difference in thermo-dependence. A high temperature sensitivity of 120 pm/℃ is achieved, and the linearity is 99.5%. The configuration is simple and has a compact size, making it a good candidate for distant temperature sensing and oil prospecting.