The invention relates to fine grain high-toughness hot stamping die steel and a preparing method thereof. Firstly, the component formula of the fine grain high-toughness hot stamping die steel comprises elements including, by weight percent, 0.28%-0.40% of C, 0.20%-1.05% of Si, 0.25%-0.85% of Mn, 3.5%-5.5% of Cr, 2.2%-3.4% of Mo, 0.8%-2% of W, 0.1%-1.0% of V and the balance Fe, and the rest of trace residual elements include S, P, N,O, H and the like. Secondly, according to the hot stamping die steel technology process, smelting is conducted after burdening, an electrode bar is formed throughcasting, after being subjected to electroslag remelting, the electrode bar is cast into a circular steel ingot, the steel ingot is subjected to high-temperature homogenizing, then, upsetting and drawing-out are conducted on a press machine, stress relief annealing is conducted, finally, ultra-refining treatment and tempering treatment are conducted, and a finished product is obtained. The size ofhot stamping die steel grains is refined to be about 1-4 microns, good toughness, high-temperature friction abrasion performance, high-temperature heat resistance, heat stability, heat fatigue performance and high heat conductivity are achieved.