A novel refractive index (RI) sensor based on microfiber (MF) coated with gold nanowires has been proposed and theoretically investigated. Compared with gold film, the gold nanowires can significantly improve the performance of the MF sensor. The influence of the diameters of gold nanowire and microfiber on the sensing properties are investigated. For analyte RI ns = 1.33, the maximum sensitivity of 5200nm/RIU (DMF = 3μm, Dwire = 120nm) and the maximum figure of merit (FOM) of 150.38RIU^−1 (DMF = 9μm, Dwire = 30nm) can be achieved. Both the sensitivity and the FOM will increase when the RI increases from 1.33 to 1.40. For ns = 1.40, an extremely high RI sensitivity of 12314nm/RIU (DMF = 10μm, Dwire = 50nm) can be obtained.