Abstract: Large-sized bars are key structural materials in fields such as energy and machinery, and the quenching process directly affects the hardness, strength and other properties of steel. However, during the quenching of large bars, uneven cooling and temperature distribution leads to insufficient hardened layer depth, stress concentration and other defects. Numerical simulation was employed to analyze the variation law of the hardened layer of 42CrMo bars with different diameters during quenching under different cooling conditions, and the influence of cooling rate on their stress distribution was revealed combining stress analysis. The results show that small-diameter bars cool faster, and the hardened layer depth increase significantly with the increase of the cooling time; the large-diameter bars cool slowly, the maximum hardened layer depth doesn't exceed 20 mm; furthermore, with the increase of the bar diameter, the radial stress, circumferential stress, axial stress and the stress gradient increase.