Aiming at the problems of high loss and low consistency of the coupling structure of the existing liquid-leakage monitoring fiber sensor with side coupling structure, a liquid-leakage sensor with cylinder defect coupling structure was proposed. The optical fiber coupling simulation is carried out by Zemax software, and the cylindrical, cone and semi-sphere structures are compared and analyzed. The cylindrical coupling structure has higher coupling rate when the insertion loss difference is small. Polymer fiber (POF) was processed to obtain a cylindrical coupling structure with insertion loss of 0.31dB.The LED lamp belt is used as the scanning light source to verify the detection ability of the optical fiber with coupling structure. When the coupling medium changes from air to water, the optical power intensity changes up to 32.8% or more.The experimental results show that the loss of the cylindrical coupling structure is significantly lower than that of the existing coupling structure, and the optical fiber sensor can effectively detect the liquid leakage.