The invention provides a process for preparing acid from phosphogypsum and fly ash and co-producing an anti-vanadium additive. The process comprises the following steps: mixing the phosphogypsum, thefly ash, the additive and a modifying agent and grinding the mixture into a raw material; feeding the raw material in a kiln and roasting the raw material in the kiln to obtain clinker; carrying out water dissolution on the clinker; carrying out solid-liquid separation; roasting the solid obtained by separation, and then processing the solid to obtain sulfuric acid; purifying the liquid obtained by separation via calcium oxide to obtain aluminium hydroxide; and preparing the anti-vanadium additive from the aluminium hydroxide and other raw materials. The phosphogypsum and the fly ash are comprehensively utilized, the process is simple, and has the advantages that environmental pollution is relieved, and the added value is high, the product aluminium hydroxide is a good raw material for preparing the anti-vanadium additive, and the co-produced anti-vanadium additive has the advantages of low production cost and good anti-vanadium effect.