The invention discloses an HRB500 reinforcing steel bar and a preparation method thereof. The reinforcing steel bar comprises the following chemical components in percentage by weight: 0.10-0.18% of C, 0.2-0.8% of Si, 1.2-1.6% of Mn, less than or equal to 0.04% of P, less than or equal to 0.04% of S, 0.001-0.005% of B and the balance of Fe and inevitable impurities. The preparation method comprises the following steps: heating a billet to 1,150-1,250 DEG C in a furnace; performing heat preservation for 100-120 minutes; taking the billet out of the furnace; rolling when the temperature is within 1,050-1,150 DEG C; rolling, and cogging for two times, wherein the billet is rolled for 14-18 times in all roughly, moderately and finely, and the total compression ratio is 64-156; cooling to 600-680 DEG C at a speed of 8-12 DEG C / s after the fine rolling is finished; finally air-cooling to the room temperature. According to the HRB500 reinforcing steel bar and the preparation method, the strength of the reinforcing steel bar is improved through tissue phase change. Compared with the prior art, the HRB500 reinforcing steel bar is smelted by using a smelting method for ordinary steel, and is subsequently cast into the billet, and meanwhile the alloy content is reduced, so that the product cost is lowered.