The invention discloses a preservative, and particularly relates to the preservative for a polycarboxylate water reducer. The preservative for the polycarboxylate water reducer comprises the following components: A, glutaraldehyde, B, dodecyl dimethyl benzyl ammonium chloride, C, dodecylguanidine hydrochloride, D, N-methyldiethanolamine, and E, sodium pyrithione. The preservative for the polycarboxylate water reducer, particularly the preservative for the polycarboxylate water reducer for the concrete can effectively prevent the polycarboxylate water reducer from going mouldy to result in deteriorating, and the quality guarantee period of the polycarboxylate water reducer can be prolonged.