The invention relates to the technical field of food making, in particular to leavening old dough juice. The leavening old dough juice is prepared by the following steps of: adding water into old dough, stirring to obtain paste, fermenting at room temperature for 8 to 12 hours, adding flour, water and a swelling agent, stirring uniformly, and fermenting at room temperature for 10 to 15 hours. A leavening composition is prepared from the following raw materials in part by weight: 500 parts of flour, 16 parts of swelling agent, 950 to 1,050 parts of leavening old dough juice, and 5 to 6 parts of alkali. A leavening process comprises the following steps of: dissolving alkali in water to obtain alkaline water, mixing the alkaline water, flour, the swelling agent and leavening old dough juice,adding a proper amount of water, mixing to obtain loose dough, and pressing by using a press flour machine. With the leavening old dough juice, the dough is not required to stand for a certain periodand can be directly steamed, the standing time for the dough is saved, and cooked flour based foods are particularly chewable and good in mouthfeel; and the leavening old dough juice is wide in application range and can be used for fermented flour based foods, such as steamed buns, steamed stuffed buns, steamed twisted rolls and bread which need fermenting.