The invention provides raw paper for thermal transfer paper and a preparation process thereof. The raw paper for the thermal transfer paper has grammage of 70 to 95 g / m2, cross file grammage tolerance of no more than 5 g / m2, apparent density of 0.8 to 0.85 g / cm3, smoothness of 45 / 36S, transverse tearability of no less than 600 mN, vertical dry tensile strength of no less than 70 N / 15mm, vertical wet tensile strength of no less than 12 N / 15 mm, air permeability of no less than 60 S, a water absorption value of 30 to 50 g / m2, a deformation rate of no more than 1.5% and dust of no more than 10 / m2. The preparation process for the raw paper for the thermal transfer paper comprises the steps consisting of pulping, mixing, beating and rewinding so as to prepare the raw paper for the thermal transfer paper, and then packaging is carried out, wherein a surface sizing solution and a filling material are added during beating. The raw paper for the thermal transfer paper prepared in the invention has the advantages of low cost, good paper formation, great apparent density, small air permeability, high tensile strength, high heat resistance, etc.