The utility model discloses a ventilate filling water gap for a continuous casting tundish and a preparation method thereof. The water gap is mainly characterized by comprising an internal ventilate core body and an external dense article made of fireproof material, wherein the ventilate core body has larger porosity and favorable ventilation property and is formed through mechanical pressure, the dense article is formed through casting vibration and takes an airtight effect, and a gap-type air chamber made of combustible substances is arranged between the ventilate core body and the dense article and is connected with an external argon gas input device through a metal conduit. The utility model has the advantages of simple production technology and low production cost due to the improved structure; the external dense article adopts a self-tightness airtight mode to save the technology for wrapping a steel casing outside the water gap, and the favorable self-tightness property can ensure the flow and the back pressure of gas to be stable in a blowing process so as to have better effect of preventing non-metal impurities in molten steel from sticking and depositing.