Abstract: By vacuum induction melting 08Cr9CoMo steel using MgO crucible, the pure iron is derusted and baked with other alloys, the charging ingredients are 0. 06% C, 1. 3% Co, 9. 0% Cr, 1. 5% Mo, 0. 02% La-Ce, the balance is iron,turn on the power with vacuum ≤10 Pa, controlling the vacuum 25 ~30 Pa during the melting period, controlling the liquid temperature 1 600 °C and the vacuum 10 Pa during the refining period, and refining for 20 min, adding 0. 02% La-Ce before casting, the final analysis result is 0. 022% C and 0.002 8% 0. The carbon deoxidation capacity under vacuum is calculated by thermodynamics, and it is significantly enhanced as vacuum reduces, the relationship between oxygen supply of the MgO crucible and vacuum is analyzed by thermodynamic calculation, when the vacuum is lower than 25 Pa, the crucible decomposes and supplies oxygen more significantly, when the decomposition rate of the MgO crucible is equal to the carbon deoxidation rate, the molten steel reaches the minimum oxygen content.