The invention discloses gear steel 20CrMoSH and a technology for improving the hardenability stability thereof. The technology comprises the procedures of converter smelting, LF refining, RH refining,continuous casting, heating and rolling. The gear steel is prepared from the chemical components, by weight, 0.19%-0.21% of C, 0.20%-0.26% of Si, 0.64%-0.70% of Mn, 1.00%-1.06% of Cr, 0.15%-0.21% ofMo, 0.015%-0.025% of S, smaller than or equal to 0.018% of P, 0.015%-0.025% of Als, smaller than or equal to 0.03% of Ni, smaller than or equal to 0.03% of Cu, smaller than or equal to 0.0015% of O and the balance Fe and unavoidable impurities. According to the gear steel 20CrMoSH which is produced through the method and has the diameter phi ranging from 28 mm to 50 mm, the hardenability J1.5 ranges from 40 HRC to 45 HRC, the hardenability J5 ranges from 37 HRC to 42 HRC, the hardenability J9 ranges from 28 HRC to 33 HRC, and the maximum discrete value of the hardenability of the steel in thesame furnace is not larger than 3 HRC. The heat-treatment deformation of a gear can be significantly reduced by means of high-stability hardenability, and therefore the meshing precision of the gear is improved. The method is low in cost and high in production efficiency, and the product has the advantages that the hardenability is stable, the gear heat-treatment deformation is small, and the machined gear is high in meshing precision and low in service life.