In this paper, an optical fiber fluid sensing system based on optoelectronic oscillator (OEO) was proposed and studied numerically. The fluid sensor head is constructed by splicing two sections of side-polished fiber (SPF) to one section of photonic crystal fiber (PCF). Fluid sample can flow continuously through the holes of PCF. The refractive index (RI) change of the fluid sample can lead to the effective RI change of the fiber, resulting in frequency change of microwave signal generated by OEO. By monitoring the oscillation frequency using an electronic spectrum analyzer (ESA), the RI of fluid sample can be measured. Thanks to the fast interrogation speed of ESAs, the measuring speed can be increased significantly compared to traditional optical fiber RI sensing systems using optical spectrometers. The sensing principle of the system was studied. The sensitivity of the proposed system was evaluated by simulation, and an RI sensitivity of ?14.20 MHz/RIU can be achieved. The results show that with proper system design, real-time RI measurement with high sensitivity can be achieved. Increasing the length of the PCF while under the premise of the fluid parameters will be the most reasonable way to improve the sensitivity. The proposed design and simulation results can provide suggestions for the fabrication and optimization of fluid sensing systems used for real-time detection and measurement of biological elements and heavy metal ions in liquid environment.