The invention relates to a high-toughness high-boron medium-chrome low-carbon wear-resisting alloy steel and a preparation method thereof. The high-toughness high-boron medium-chrome low-carbon wear-resisting alloy steel comprises the following chemical components by weight percentage: 0.20-0.5% of C, 5-12% of Cr, 0.5-1.2% of Si, 3.5-5.5% of Mn, 0.3-2.8% of B, 0.3-2.1% of Cu, 0.2-0.5% of Ti, 0.05-0.25% of Ca, 0.03-0.3% of Ce, 0.02-0.18% of N, 0.05-0.3% of Nb, 0.04-0.09% of Al, 0.1-0.5% of SiMgRe, 0.04-0.13% of K, less than 0.03% of S, less than 0.04% of P and the balance of Fe and inevitable impurity elements. The preparation method comprises the following steps of: adding a copper plate, silicon iron and ferromanganese iron after steel scrap and chromium iron are melted; after the component adjustment before furnace is qualified, increasing the temperature of the melt to 1560-1620 DEG C, adding a calcium-silicon alloy and aluminum for deoxidation, and orderly adding ferrotitanium and ferroboron, melting, and then discharging; putting a composite inoculant composed of granular rare-earth magnesium alloy with granule size being less than 12mm, metal cerium, Si3N4, VN, Nb and K at the bottom of a steel ladle after baking, and performing inoculation treatment on the molten steel by a rush-into-ladle method, wherein the casting temperature of the molten steel ranges from 1400 DEG C to 1450 DEG C; preserving the heat of castings for 4-6 hours at 700-780 DEG C, and then carrying out subcritical air quenching to obtain the alloy steel.