The lasing threshold of evanescent-wave pumped whispering_gallery_mode （WGM） fiber lasers has been studied. Quartz fibers of different diameters were immersed in a mixed solution of ethanol and ethylene glycol doped by rhodamine 6G dye molecules, the refractive index of the solution was variable and lower than that of quartz fiber. We find that the dependence of lasing threshold on the refractive index of the mixed solution differs for different diameter of quartz fiber. With the increase of refractive index of the mixed solution from 1362 to 1432, the threshold energy decreases slowly and monotonicly for fibers of diameters larger than 296 μm, but increases for fibers of diameters smaller than 93 μm; for a fiber of diameter 197 μm, the threshold energy decreases slowly at first, and then increases sharply, there is an optimum refractive index of the mixed solution which matches a minimum threshold energy. Based on the theory of evanescent-wave pumped WGM fiber laser, we deduce a formula of lasing threshold, the calculated curves of lasing threshold are well consistent with the experimental data.