The invention relates to a preparation method of sour dough. The preparation method comprises the following steps of (1) uniformly mixing flour, water and dry yeast so as to obtain dough; (2) repeatedly and uniformly kneading and pressing the dough for 5-10 minutes; (3) performing primary fermentation; (4) performing secondary fermentation; (5) cutting the dough after the secondary fermentation into cuboid dough, and processing a blind hole in the surface of the cuboid dough; (6) refrigerating the cuboid dough; (7) taking out the refrigerated dough, and adding baking soda powder in the blind hole, wherein the proportion of the baking soda powder to the flour is (0.08-0.1) to 100; and (8) uniformly wrapping the dough added with the baking soda powder with a preservative film, and refrigerating the dough wrapped with the preservative film for standby application. The baking soda powder is placed in the blind hole, so that when the sour dough needs to be used, the refrigerated sour dough is taken out, the preservative film is torn away, and the flour, the sour dough and the water are mixed according to the proportion of the flour to the sour dough to the water being 100 to 1 to (50-60); the baking soda powder in proportion is placed in the blind hole, so that fermented dough can automatically offset lactic acid bacteria in the sour dough, the sour dough is convenient to use, and the mouth feel is improved.