The invention belongs to the field of chemical engineering and biomass energy, particularly relates to the field of biomass utilization and more particularly relates to a method for preparing biogas.The method comprises the following steps: carbonizing a raw material containing rich lignin at 400-600 DEG C for 10-90 min to obtain a lignin carbonized material, and separating out gas. The inventionfurther relates to a method for preparing activated carbon, the prepared activated carbon and a co-production method for the biogas and the activated carbon. Biogas with high fuel value and the activated carbon with large specific surface area are obtained with the methods, and with adoption of the methods, effective degradation and regeneration of plant straw are realized, and environmental pressure and energy pressure are relieved.