In this work, we present a multi-mode resonator based on SU-8 polymer and experimentally verify that the resonator showed mode discrimination can be used as a sensor with high performance. According to field emission scanning electron microscopy (FE-SEM) images, the fabricated resonator shows sidewall roughness which is canonically considered to be undesirable after a typical development process. In order to analyze the effect of sidewall roughness, we conduct the resonator simulation considering the roughness under various conditions. Mode discrimination still occurs even in the presence of sidewall roughness. In addition, waveguide width controllable by UV exposure time effectively contributes to mode discrimination. To verify the resonator as a sensor, we perform a temperature variation experiment, which results in a high sensitivity of about 630.8 nm/RIU. This result shows that the multi-mode resonator sensor fabricated via a simple process is competitive with other single-mode waveguide sensors.