An optical model of side-polished photonic crystal fiber (SP-PCF) is presented firstly, then simulations and experiments are carried out to investigate the optical characteristics of the SP-PCF under different conditions, i.e., different side-polished depths and different covered materials. The results of both the simulations and the experiments show that there exists a side-polished sensitive area. When the polished depth is changing in the area, the refractive index of the covered material has obvious influence on the the effective refractive index of fundamental mode of the SP-PCF. According to the results, SP-PCF could be used as ambient refractive index sensor, and it has potential applications as miniature bioorchemical sensors.