The temperature cross-sensitivity characteristics of a singlemode-multimode-singlemode (SMS) fiber structure packaged by a shell are studied both theoretically and experimentally. By theoretical investigation, we found that the temperature sensitivity of a SMS structure is mainly determined by the thermo-optic effect (TOE) of the cladding of the multimode fiber (MMF). Meanwhile, the TOE of the MMF core, thermal expansion effects (TEEs) of the MMF core, and the packaging material also influence the ultimate sensitivity, and the magnitude of their effects depends on the refractive index of the MMF cladding. Among them, the TEE of the packaging material, inducing an axial strain, is considered to be the second main factor. A temperature sensor based on a packaged SMS structure is designed and investigated to experimentally verify the theoretical findings. The experimentally measured temperature sensitivity of the sensor is -453.4 pm/°C, which agrees well with the theoretical prediction.