The invention provides a method for preparing acid and cooperatively producing a resin-based flame-retardant composite material. The method comprises the following steps that ardealite, red mud, an additive and a modifying agent are mixed and ground into a raw material, the raw material is fed into a kiln, roasting is performed to prepare clinker, water milling and dissolution are conducted on theclinker, solid-liquid separation is performed, solid obtained through the separation is roasted and then processed to prepare sulfuric acid, liquid obtained through the separation is processed to obtain aluminum hydroxide, and then the aluminum hydroxide, microencapsulated red phosphorus, dimethyl methylphosphonate, a resin matrix, a curing agent, an accelerant and a reinforcing material are prepared into the resin-based flame-retardant composite material. The ardealite and the red mud are comprehensively utilized, the material has the advantages of reducing environmental pollution and beinghigh in added value and valuable ingredient utilization rate, the prepared aluminum hydroxide is high in purity and is a good raw material serving as a flame retardant in the resin-based flame-retardant composite material, and the prepared resin-based flame-retardant composite material has the advantages of being simple in process, good in flame-retardant performance, good in physical and mechanical properties and low in production cost.