The invention discloses a phosphogypsum composite autoclaved brick, and the formula of a green body (content by weight) contains 60%-85% of phosphogypsum, 10%-20% of construction garbage and 1%-8% of additive. The construction garbage is one or mixture of broken red bricks, waste concrete, waste ceramic tiles and waste glass, and the additive is mixed material of a plurality of groups of raw materials of sodium citrate, methyl cellulose, FDN, and quick lime. The invention further discloses a preparation method of the phosphogypsum composite autoclaved brick, which comprises the procedures of breaking, ball milling, material preparing, stirring, aging, wheel grinding, pressing, autoclaving and the like. The composite autoclaved brick has the advantages of high utilization rate of the solid waste residue-phosphogypsum and the construction garbage, large utilization amount, simple production process, low cost, economy and environmental protection, and the brick body can meet therequirements of national standards on bearing materials of wall bodies. The content of solid wastes of the phosphogypsum, the construction garbage and the like in the formula is as high as 95%, thereby changing wastes into treasures and belonging to a novel environment-friendly construction material.