The invention provides a nano-composite water retaining agent and a preparation method thereof. The nano-composite water retaining agent is prepared from the following components and raw materials inparts by weight: 1-3 parts of nanometer materials, 14.2 parts of acrylamide, 6.2 parts of 2-acrylamide 2-methyl-propionic acid, 3 parts of methacrylate, 0.05 parts of initiator, 4.3 parts of sodium hydroxide, and 527-530 parts of deionized water. The preparation method comprises the following steps: weighing a nanometer material, dissolving into the deionized water, and ultrasonically dispersing,to obtain a nanometer material dispersion liquid; adding nanometer material dispersion liquid, acrylamide, 2-acrylamide 2-methyl-propionic acid, methacrylate, initiator, sodium hydroxide and deionizedwater into a reaction kettle, stirring and heating; adding the initiator into the heated solution, keeping at constant temperature, reacting under stirring, and then adding sodium hydroxide and deionized water, to obtain sticky liquid, namely the nano-composite water retaining agent. The nano-composite water retaining agent is good in cement adsorbability, can remarkably improve the adhesive aggregation, segregation resistance and bleeding resistance performances of cement-based grouting materials, and is simple in preparation technology.