Abstract: In this article, we proposed an optical fiber sensing system for measuring vibration, strain and temperature based on the multimode interference of single-mode-multimode-single-mode (SMS) fiber structure, because the output optical power of the SMS fiber structure changed with the external environment parameters, such as vibration (frequency and amplitude), strain and temperature. From the variation of the output optical power, the strain on the multimode fiber could be obtained. The frequency and amplitude of the vibration and temperature of the SMS fiber structure could be obtained when FFT or Hilbert transformation was employed. With the calibration of the system, some cases of vibration and temperature change were measured with the error of less than 0.7 Hz and 5.0%, respectively. The SMS optical fiber sensing system can be used for multi-parameter measurements with extremely simple structure and low cost.