The invention relates to a preparation method for a cationic silicone/acrylic emulsion alkali resistant seal primer. The basic formula of the seal primer is as follows: 30% to 50% of emulsion, 4% to 8% of titanium dioxide, 5% to 12% of ground limestone, 5% to 10% of kaolin, 1% to 3% of talcum powder, 2% to 6% of coalescing agent, 1% to 5% of propylene glycol, 0.5% to 3% of thickening rheological agent, 2% to 5% of assistant and 20% to 35% of deionized water. The seal primer has the advantages of strong permeability, excellent weather and water resistance, good adhesion with substrate and paint, good self cleanness and environmental friendliness, excellent sealing performance, excellent alkali resistance and higher alkali and salting-out resisting performance. Therefore, the seal primer is suitable for being coated on the surfaces of external walls, internal walls, cement walls, concrete, gypsum boards, asbestos plates and the like.