The invention relates to a Co-Fe alloy/carbon sphere composite microwave absorbent and a preparation method thereof. Carbon spheres are taken as a core, the outer sides of the carbon spheres are coated with Co-Fe alloy particles, and the outer sides of the Co-Fe alloy particles are coated with a layer of graphite, so that a multi-stage core-shell structure is wholly formed. The preparation method of the composite microwave absorbent comprises the following steps: at first, incompletely carbonized polysaccharide microspheres are prepared from glucose and a dispersant, then the polysaccharide microspheres are impregnated by a cobalt ion and iron ion mixed solution, and finally, the obtained mixture is placed in an inert atmosphere to be subjected to heat treatment at high temperature, so that graphitization of carbon spheres and reduction of metal ions are performed simultaneously, and the obtained composite microwave absorbent combines the advantages of both a carbon material and magnetic metal powder, has the characteristic of light weight while having favorable microwave absorption performance, and can be widely applied to various environments and fields.