A method for anaerobic digestion of easily acidified wastewater (substance) to produce methane comprises the following steps: (1) directly adding the easily acidified wastewater into a fully mixed anaerobic digestion reactor as a fermentation substrate; (2) adding the inoculum into a fully mixed anaerobic digestion reactor; (3) adding granular activated carbon into a fully mix anaerobic digestionreactor; (4) carrying out anaerobic digestion reaction, and before that reaction proceeds, introducing nitrogen gas into the reactor to ensure that there is no oxygen in the system; Then at the speedof 100r/min and the temperature of 30-50 DEG C, carrying out reation. During the process of the reaction, the pH value does not need to be adjusted, and the time of anaerobic digestion is controlled to be 5-20 days. Methanogenesis of easily acidified wastewater (substance) was realized by adding granular activated carbon into digested substrate, which was produced in food processing industry, vegetable processing industry and catering industry, and then the acidified wastewater (substance) was digested.