An in-fiber Michelson interferometer based on multimode interference is proposed and fabricated. The proposed interferometer comprises a section of multimode fiber (MMF) fusion-spliced with single mode fiber (SMF). When light enters the MMF from the SMF, multiple high-order modes are excited by core mismatching. These high-order modes and core mode propagate in the MMF and couple with each other to form intermodal interference. The interference fringes of the proposed sensor samples are clear with high contrast. Liquid refractive index (RI) sensing and temperature measurement experiments demonstrate that a liquid refractive index sensitivity of -92.43 dB/RIU is achieved for the liquid RI range from 1.3333 to 1.3796 RIU, and a temperature sensitivity of 0.01 dB/℃ is obtained for a water temperature range of 25-75 ℃. The proposed sensor is easy to fabricate and cost-effective. The probe structure of the proposed sensor has promising applications in biomedical and petrochemical fields.