The invention relates to the field of water reducing agents, and discloses an early-strength polycarboxylate superplasticizer and a preparation method thereof. The early-strength polycarboxylate superplasticizer is prepared from 10 to 15 parts of methylallylpolyoxyethylene ether, 25-30 parts of acrylic acid, 2-3 parts of sodium sulfonate methacrylate, 1-2 parts of an initiator, 4-5 parts of 1,2-benzisothiazole-3-one, 0.4-0.5 part of nanometer gallium oxide, 0.6-0.8 part of a solvent, and 50-60 parts of water. The preparation method comprises the following steps: mixing nanometer gallium oxide and a solvent, adding 1,2-benzisothiazole-3-one, and adding the mixture into a polycarboxylic acid polymer synthesized by other components to obtain the polycarboxylate superplasticizer. The preparation method has the advantages that the nanometer gallium oxide contains a microcosmic porous structure and can be better blended with 1,2-benzisothiazole-3-one after being mixed with a solvent, so a product adsorbing free carboxyl groups is obtained, the content of the free carboxyl groups capable of being complexed with calcium ions in a liquid phase is reduced, hydration is accelerated, coagulation time is shortened, water consumption is reduced, water film layers are decreased, and aggregate-cement interface structure is changed, so the early strength of concrete is better improved.