The invention discloses a silane coupling agent modified polycarboxylate superplasticizer and a preparation method thereof, belonging to the field of cement additives. The preparation method comprises the following steps: adding a certain amount of acrylic acid into a polyether macromonomer solution, and stirring uniformly; adding a double bond-containing silane coupling agent; stirring the solution to a clear state and continuously stirring; adding an oxidizing agent, an initiator solution and a small monomer solution while keeping the stirring state; after the stirring, curing for 1-2h; adding liquid caustic soda for neutralization and stabilizing the pH value at 6; and adding a silane coupling agent without double bond to obtain a finished product of silane coupling agent modified polycarboxylate superplasticizer. In the invention, a functional group with a siloxane structure is introduced into the main chain of polycarboxylate molecule, and an anchoring ability in chemical bonding strong interaction with cementing material particles is provided for the branch chain of the comb-shaped polycarboxylate superplasticizer; and with the increase of the anchoring ability, the adaptability and slump loss resistance of the superplasticizer can be remarkably enhanced, and the application of the superplasticizer in the field of mineral micropowder dispersion is expanded to a certain degree.