The invention provides a ground-layer fireproof coating for an ancient building or a woody structure, fireproof mud and a preparation method thereof, and belongs to the field of fireproof coatings. The ground-layer fireproof coating is prepared from fireproof mud and fireproof liquid which are matched for use, wherein the fireproof mud is prepared from aggregate, biomass powder, graphite, zinc borate, nano silicon dioxide and a toughening agent; the fireproof liquid is prepared from an inorganic silicon flame retardant, a waterproof agent, a wetting agent, a dispersing agent and a defoaming agent. The ground-layer fireproof coating provided by the invention is strong in adhesive force, high in hardness, good in toughness and weathering resistance, difficult in cracking and dropping, can resist burning of high-temperature raging fire with the temperature being hundreds of degrees centigrade, can be gradually solidified like stones when meeting an open fire, can insulate high temperatureand protect woods from being ignited, so that the building can be protected from being burnt down.