A conical cross-section gold nanoholes array is fabricated on a multi-mode optical fiber tip with an electron-beam lithography process. Underneath the conical gold nanoholes, nanocavities are etched into the substrate fiber glass to increase the light–matter interaction volume. Localized surface plasmon resonance of the fabricated fiber optical device is characterized by measuring the optical transmittance through the fiber tip in the visible and near infrared spectral range. The fiber optical device is used as an index of refraction sensor to detect isopropyl alcohol, ethanol, and methanol. A largely enhanced sensitivity of 652  nm per refractive index unit due to the conical cross-section gold nanoholes and the nanocavities in the fiber glass substrate is observed. Additionally, the response and recovery times of the fiber-tip plasmon sensor are measured for the first time.