The invention discloses a process for preparing a sulfonic acid curing agent for casting from industrial waste, i.e. sulfonic acid mother liquor, which comprises the following steps of: pre-treating the sulfonic acid mother liquor, adding 98% concentrated sulfuric acid 175 to 250 parts in dimethylbenzene 100 parts, stirring, heating to 110 to 140 DEG C, keeping the temperature and reacting for 1 to 3 h, decreasing the temperature below 70 DEG C, adding water and alcohol solvent 40 to 105 parts slowly, adding the pre-treated sulfonic acid mother liquor 70 to 350 parts, stirring for 0.5 to 1 h, decreasing the temperature below 40 DEG C, adding hydrogen peroxide 2 to 3 per mill, and stirring uniformly to obtain the final product. The sulfonic acid curing agent prepared by the process keeps the excellent performance of original curing agent, and is improved in the selection of use range, cured strength, viscosity, odor, decrease of production cost, etc. by different degrees. Particularly, the sulfonic acid curing agent has very strong competition advantages in environment protection, low viscosity and low cost.