The invention relates to a glass fiber reinforced polycarbonate resin and a preparation method thereof in the technical field of macromolecular compounds. The glass fiber reinforced polycarbonate resin comprises the following components in percentage by mass: 50 to 78 percent of polycarbonate, 1 to 10 percent of low molecular weight resin, 20 to 40 percent of glass fiber, and 0. 1 to 0.5 percent of antioxidant. The preparation method of the composition comprises the following steps of: mixing the polycarbonate, the low molecular weight resin and the antioxidant in a medium speed mixer; and putting the mixture obtained in the step 1 into a double-screw extruder, laterally feeding the glass fiber, and performing melt extrusion and granulation to obtain the resin. The low molecular weight resin with good infiltrative property with the glass fiber is added into the glass fiber reinforced polycarbonate resin, so that the infiltrative property of the glass fiber and the liquidity of the material are improved, the glass fiber is not easily leaked, the surface effect of the product is improved, and the comprehensive performance of the product is further promoted.