We proposed and experimentally demonstrated an all-fiber sensor for measuring bend with high sensitivity based on a ring core fiber (RCF) modal interferometer. The sensor was fabricated by splicing a segment of RCF between two pieces of multimode fiber (MMF) and single-mode fiber (SMF) at the ends of the MMF as lead-in and lead-out. Due to the first segment of the MMF, the transmitted light is coupled into the ring core, silica center, and cladding of the RCF, exciting multiple modes in the RCF. By the modal interferences in the structure, bending sensing can be realized by interrogating the intensity of the interference dip. Experimental results show a high bending sensitivity of -25.63dB/m-1 in the range of 1.0954m-1 to 1.4696m-1. In addition, the advantages of the bend sensor, such as small size, low temperature sensitivity, and simple fabrication process, can be used for curvature measurement in building health monitoring.