The invention discloses a preparation method for a waterproof brick. The method comprises the following steps: 1) taking 3 parts of methane-siliconic acid, 2 parts of caustic soda and 10 parts of water by weight parts, mixing and then heating to 85-95 DEG C, thereby acquiring a sodium methyl silicate solution; 2) adding 0.3%-0.5% of citric acid, 0.05%-0.3% of triisopropanolamine and 0.01%-0.2% of aluminum sulfate on the basis of the weight of the sodium methyl silicate solution acquired in the step 1) and then adding 30 times of water for diluting; and 3) taking a common fired clay brick, completely soaking the clay brick in the solution acquired in the step 2) for 15 minutes, taking out after soaking and then naturally airing. The preparation method disclosed by the invention is simple in process, is based on the common clay brick manufactured according to the present process and is economical and environmentally friendly; the waterproof brick is capable of effectively achieving the waterproof, mold-proof, anti-freezing, anti-alkali and anti-weathering effects; the pored crystals in the brick are stable; the waterproof property is not greatly reduced following the time; the service life of the wall is effectively prolonged; the crystals are filled in the brick, so that the firmness of the brick is promoted and the brick is difficult to break.