The invention relates to a new macromolecular sulfur-containing organophosphorus flame-retardant compound of the following formula and a preparation method thereof, wherein, n is larger than 1. The preparation method comprises the following steps: in the presence of nitrogen, carrying out heating reaction on equimolar phenylphosphonothioic dichloride and glycol in the presence of catalysts for a period of time, then carrying out purification and drying on the reactant to obtain polyphenyl thiophosphate gylcol ester. The prepared compound has high flam resistance, stable physical and chemical characteristics, and good intermiscibility with high polymer materials, and is suitable for the flame-resistant treatment of polyurethane and other materials. The method has the advantage of simple process and is easy to realize industrial production.