The invention discloses a preparation process of an anti-DC transformer iron core. The preparation process comprises the following steps that a, mounting is conducted, specifically, the annular iron core is fixed to a clamp of an electric spark cutting machine; b, parameters of electric spark line cutting are set, specifically, the current is 1.5-3 A, the cutting speed is 1.0-1.5 mm/min, the diameter of an electrode wire is phi 0.16 mm, and the unilateral discharge gap is 0.01 mm; c, cutting is conducted, specifically, the electric spark cutting machine is started, thus a cutting tool conductscutting according to the linear trajectory passing through the circular center of the annular iron core, and an air gap is cut in the annular iron core; and d, an insulating sheet is mounted, specifically, the electric spark cutting machine is stopped, the annular iron core is removed from the clamp of the electric spark cutting machine, the insulating sheet with the required thickness is sandwiched in the machined air gap, and the size of the air gap is controlled. The preparation process is simple, the skill requirements for production workers are lowered, and the manufactured single-air-gap iron core has high consistency and is suitable for mass production.