Dice purple potato, steam it, and press it into mud. Add baking powder to the flour, and then add sugar. Then add the purple sweet potato paste and knead well. Add proper amount of purple potato yeast water into purple potato powder. Knead the purple potato powder into dough and wake it in a warm place for 45 minutes. Add baking powder, sugar and yeast water to the other part of the flour and knead it into dough. Put the kneaded dough in a warm place and wake it for 45 minutes. Two pieces of dough are respectively rounded, flattened, put together from top to bottom, and steamed in a pot.