The invention discloses a method for preparing vasopressin tannate. The preparation method of the vasopressin comprises the following step: sequentially carrying out reverse-phase purification and reverse-phase desalination on a vasopressin crude product solution by a high performance liquid chromatography, so as to obtain the vasopressin tannate, wherein packing of the high performance liquid chromatography, is a polystyrene-divinylbenzene (PS-DVB) copolymer. By combination of reverse-phase purification and reverse-phase desalination, the latest application of the polymer packing polystyrene-divinylbenzene is designed; and the vasopressin tannate can be prepared on a large scale.