Fresnel reflection-based optical fiber sensor for quasi-distributed refractive index (RI) detection using the optical time domain reflection technique as an interrogation method is proposed. Ambient RIs at different positions along the long fiber link can be obtained by monitoring the Fresnel reflection signals. The experimental results show that the measurement range can reach 16 km, and the refractive index sensitivity is from 38.785 dB/RIU to 305.430 dB/RIU (RIU means refractive index unit) when the RI is from 1.3486 to 1.4525.