The invention discloses a multifunctional inorganic composite foam concrete modifier and a preparation method thereof. The multifunctional inorganic composite foam concrete modifier is prepared from inorganic functional material nano silicon powder, mineral powder, expansive quick-hardening cement, rubber powder, a water reducing agent powder and a water repellent powder. The preparation method comprises the following steps: sequentially adding all the materials into a stirrer in parts by weight, mechanically stirring while adding the raw materials, and uniformly mixing, thereby obtaining themultifunctional inorganic composite foam concrete modifier; and then mixing and foaming according to the ratio of 0.80 part of a master batch to 0.20 part of the multifunctional inorganic composite foam concrete modifier to prepare a high-performance foam concrete product. The compressive strength of the foam concrete added with the modifier reaches 1.45 times of the industrial standard, and the foam concrete has the advantages of light weight, hydrophobicity, no cracking, flame retardance, heat preservation and insulation, sound insulation, the same service life as a building main body structure and the like, and is simple to prepare and convenient to operate and produce.