The invention relates to the technical field of the utilization of manganese-containing waste liquid obtained by producing hydroquinone, in particular to a method for preparing co-product of manganese phosphate and potassium sulfate as well as ammonium phosphate by manganese-containing waste liquid obtained by producing hydroquinone. The method mainly comprises the steps of: firstly, leaching the manganese-containing waste liquid obtained by producing hydroquinone by a leaching machine, and obtaining filtrate for distilling and filtering; carrying out deep processing on a filter cake, and obtaining a manganese phosphate product; and then carrying out reduced pressure distillation on the filtrate, respectively carrying out further deep processing, and obtaining a potassium sulfate product and a ammonium phosphate product. The method leads the waste liquid to be cleanly removed so as to reduce the pollution of the waste liquid for the environment, and can be used for preparing the products of the valuable manganese phosphate, potassium sulfate and ammonium phosphate, thus not only being remarkable in social benefit and economic benefit, but also being simple in technique and production equipment; therefore, the invention is a more practical processing and utilizing method.