A new kind of low threshold optical fiber laser has been fabricated by immersing a fused silica optical fiber into a low refractive index dye solution. Pumped by evanescent wave along the fiber axis, dye gain is confined in the evanescent field of whispering gallery mode (WGM) of the fiber, which leads to a low pumping threshold for lasing emission. The concentration effects in liquid dye lasing emission have been studied via a WGM fibre laser under the pumping energy of 50 μJ. The results show that with the increase of rhodamine 6G dye concentration in ethanol alcohol, lasing wavelength is red shifted and lasing wavelength range is widened. The experimental observations are well explained by the γ(λ) curve, a ratio of molecular density for the upper and total lasing energy levels, which is deduced from the four-energy level model of dye lasing from a WGM fibre laser. The low threshold optical fiber laser provides a convenient way to study liquid dye lasing phenomenon.