The invention relates to an acid-adjusting hydrolysis process for preparing levulinic acid. The raw materials are ground, sieved and combined, and the concentration of the catalyst of sulfuric acid is adjusted to reduce the initial concentration of the reacted acid, thus the reaction can be carried out more moderate and environmental-friendly, and more importantly, the yield of the levulinic acidprepared with the acid-adjusting hydrolysis process can be greatly improved by 57.95 percent on average. In the invention, the waste agricultural crops such as the core straw, the rice straw and the rice husk which has the lowest utilization ratio are combined, thus the content of the cellulose in the raw materials are improved, and the carbonization to the rice husk from the acid is reduced. Thecellulose can be thoroughly hydrolyzed in the reaction process through adjusting the concentration of the sulfuric acid in the reaction process, while the slag from other similar hydrolysis reaction contains part of the cellulose which is not hydrolyzed in the current time. The invention can avoid the raw material carbonization and the equipment damage which are resulted from the over high concentration of the acid.