The invention relates to a modified carbon fiber magnesite board prepared with industrial waste acid and a preparation process thereof and solves the technical problems that a process in the prior arthas complex steps, has to greatly increase production cost for improving breaking strength and anti-impact strength of magnesite material, and is high in dry shrinkage rate and wet swelling rate. Themodified carbon fiber magnesite board is composed of three layers, which are tightly successively arranged from the top to the bottom: a first glass fiber net, a carbon fiber magnesite board inner layer and a second glass fiber net. The carbon fiber magnesite board inner layer is prepared from, by weight, 0.6-2.0 parts of carbon fibers after surface oxidizing treatment, 40-50 parts of sawdust, 160-180 parts of magnesite powder, and 580-700 parts of the industrial waste acid being 8-16% in concentration. The invention also provides a preparation method of the modified carbon fiber magnesite board. The modified carbon fiber magnesite board is extensively applied to the field of building materials.