In order to simplify the design and fabrication of optical fiber liquid level sensor, we propose an in-fiber Michelson interferometer based on fiber-core mismatching intermodal interference, which consists of a single mode fiber (SMF) fusion-spliced with a section of thinned fiber (TF). The fusion point of the SMF-TF acts as a fiber coupler to excite the high-order cladding modes. The core mode and high-order cladding modes are reflected by the end face of the TF and transmitted to SMF to generate intermodal interference. The output interference fringes are clear and in high contrast, which are sensitive to the change of environment liquid level. The sensing characteristics of a sensor sample with 12-mm-long TF to liquid level and temperature are investigated. The experimental results show that the reflection dip wavelength linearly changes for a liquid level variation of 0-9 mm, and the sensitivities to water and NaCl solution with mass faction of 4.7% are －0.116 and －0.129 nm/mm; the temperature sensitivity to water is 0.038 nm/℃ in the temperature range of 20-80 ℃. The sensor has advantages of simple structure, easy fabrication, and low cost, which offers the prospective application in petroleum industrial and chemical industry.