The invention discloses a preparation method of a green composite nano magnetic material for effectively removing heavy metals in an environment. The invention relates to the field of environmental functional materials, and particularly relates to a preparation method of a composite nano magnetic material. The invention aims at solving the technical problems that a current method of a biochar improved composite material is fussy, raw materials are expensive and meanwhile, a modified or improved material still has a certain risk of the secondary pollution to the environment and the like. The method comprises the following steps of 1, carrying out the pretreatment of biomass waste; 2, carrying out the thermal conversion from biomass to biochar; 3, carrying out the mixed culture of the biochar and ferric iron in a liquid phase; 4, carrying out a reduction reaction on suspension liquid by utilizing a liquid-phase deposition method, so as to obtain a nano zero-valent iron loaded carbon material, flushing, carrying out suction filtration, and oven-drying to obtain the green composite nano magnetic material. The green composite nano magnetic material is used for removing the heavy metalsin the environment, within a short time, has better removal capacity for six common heavy metals in the environment, and meanwhile, has higher magnetism; the removal efficiency reaches 95 percent or above, and the recycling is facilitated.