The invention relates to a ventilation top water gap for a continuous casting tundish and a preparation method thereof. The ventilation top water gap comprises two parts: a ventilation core formed by internal mechanical pressing and a dense piece formed by external pouring, wherein a slit type air chamber formed after combustible materials are burnt is arranged between the ventilation core and the dense piece; the ventilation core comprises the following raw materials in percentage by mass: 8-15% of corundum materials, 80-83% of corundum materials, 5-10% of other additives and 3-4.5% of binding agents; the dense piece comprises sub-white corundum and white corundum or magnesium aluminate spinel and corundum materials; the ventilation core is integrated with the dense piece after being fired; and the air chamber is connected with argon gas through a metal pipe. The water gap prepared by the invention has simple processes, lower cost and good air permeability, and the adhesion and deposition of impurities in the molten steel at the water gap can be obviously reduced; and the water gap adopts a self-sealed airtight manner, thereby saving the cost of sealing the steel shell of the traditional ventilation water gap, and ensuring that the gas flow and back pressure in the blowing process are not reduced.