The invention discloses composite modified biomass activated carbon for degrading formaldehyde and a preparation method of the activated carbon and belongs to the field of solid waste resource utilization. The composite modified biomass activated carbon comprises biomass waste, a composite modification agent, a composite grinding aid, phosphoric acid, absolute ethyl alcohol, hydrochloric acid andwater. The biomass waste comprises one or more of fruit shells, kernels and straw; the composite modification agent is a mixture of Bayer-process red mud and coal gangue. The Bayer-process red mud, the coal gangue and the biomass waste material are compounded for preparing the composite modified biomass activated carbon, a metal oxide in the mixture of the Bayer-process red mud and the coal gangueis used for modifying the biomass waste material, the service life of the activated carbon is prolonged, the mechanical strength of the activated carbon is improved, the production cost of the activated carbon is decreased by 50-60%, the high additional value application range of the Bayer-process red mud, the coal gangue and the biomass waste material is enlarged, and a new idea of using waste to treat dangers is achieved.