In this study, a novel high-reliability liquid leakage sensor with a V-shaped defect coupling structure (VDCS) is proposed and experimentally demonstrated using polymer optical fibers (POFs) based on a light-emitting diode (LED) side-coupled light source. Liquid leakage detection is achieved with changes in the refractive index as the coupling medium transforms from air to liquid. The LED lamp both provides a light source and assists positioning. The coupling efficiency of the POF liquid leakage sensor varies with the depth and angle of the VDCS. Experimental results show that the coupling efficiency of the POF leakage sensor is high when the VDCS depth is 0.5 mm and the inclination angle is 60°. Compared to an existing leakage sensor with a hole-shape defect coupling structure (HDCS), the VDCS has a faster response time and higher reversibility. The average reliability of the liquid leakage sensor is increased to 5.92 μW/mL. This research provides a powerful structural reference for POF side-coupling leak measurement and can also be applied to the fields of gas and humidity sensing.