Abstract: By using optical time domain reflection (OTDR) to measure the intensity of the reflected light intensity of multi-mode fiber (MMF), we propose a fiber optic sensor which can detect the leakage of liquid, and carry on the theoretical simulation and experimental study. As we all know, OTDR has the characteristics of small size, precision, intelligent, easy to move, and can effectively avoid the problem that the conventional, complicated and heavy optical fiber sensor devices cannot move. Use a certain concentration of hydrofluoric acid (HF) solution to etch multimode fiber as sensor components, which is easy to design and manufacture. When the core diameter of multimode fiber decreases to a certain degree, we expose it in the air or liquid under test. The change of the refractive index of the surrounding medium will affect the mode field distribution of the optical field and the intensity of the transmitted light. Therefore, the structure can effectively detect the leakage of external environment liquid. We design and make two sensors in this paper. The first sensor is only with a sensing structure. Connect OTDR and the long fiber with the sensing structure. Set the scanning pulse width of 30ns, the scan time of 3min, the strength of the light pulse signal about 15dB, the scan distance of 5km on the OTDR. Take water as test object. First, the sensing structure is exposed in the air, reflection loss curve of OTDR is 2.893dB, when it detects water，reflection loss curve of OTDR was 0.631dB. Therefore, when it detects water, the loss of OTDR is decreased, and the intensity of the reflected light is large. The experimental results are in agreement with the simulation results. The second sensor with two sensing structures is made into the distributed multiplex sensor for detecting water of different regional environment, the two sensor structures can detect water sensitively, which prove the multiplexing characteristic of the sensor. Therefore, the liquid leakage detection optical fiber sensor can be widely used in engineering practice.