We proposed and demonstrated an optical fiber temperature sensor based on rare-earth-doped double-fiber peanut (RDDFP). We used fibers doped with rare-earth elements to fabricate a peanut-shaped structure that can realize high temperature sensitivity. The sensitivity of temperature to the cladding mode and core mode interference are used and the strong optothermal effect of rare-earth element ions is combined in the proposed structure. We have theoretically and experimentally investigated and compared the mode interference and thermosensitive effect of the erbium-doped double-fiber peanut (EDDFP) and ytterbium-doped double-fiber peanut (YDDFP). Experimental results show that comparing with a single-mode fiber peanut, RDDFP exhibits higher temperature sensitivity because of its stronger optothermal effect. The temperature sensitivity of EDDFP and YDDFP are 1286 pm/℃ and -2343 pm/℃, respectively. The optical fiber temperature sensor based on RDDFP has the advantages of high sensitivity, high repeatability, all fiber, simple fabrication, compact structure, and so on. It has good application prospects in the fields of power systems, architecture, aerospace, and ocean development.