The invention relates to medium-infrared gain sulfide optical fibers and a preparation method thereof, and belongs to laser gain optical fibers and a preparation method thereof. According to the present invention, the optical fiber core material is a rare earth ion doped Ga-Sb-As-S glass, the optical fiber cladding material is a Ga-Sb-As-S glass, the refractive index of the optical fiber core glass is greater than the refractive index of the cladding glad, the chemical formula of the Ga-Sb-As-S glass is Ga(1-x-y-z)SbxAsySz, x is 0.26-0.32, y is 0.02-0.06, z is 0.55-0.65, the rare earth ion is one selected from Dy<3+>, Er<3+> and Tm<3+>, and the molar concentration of the doped rare earth ions is 0.05-1%; the optical fibers are prepared through a rod tube method, wherein the fiber core glass and the cladding glass are respectively prepared into the thin rod and the bushing, the fiber core glass thin rod is inserted into the cladding glass bushing, and drawing is performed to obtain the optical fibers having the final size; and the gain sulfide optical fibers have strong luminescence at a wavelength of 2.5-5 [mu]m, the luminescence quantum efficiency is greater than 70%, and the gain sulfide optical fibers can be adopted as the core gain medium of the low-cost and compact medium-infrared optical fiber laser.