The invention discloses a borate coupling agent and a preparation method thereof. The borate coupling agent is prepared by the following method: mixing boric acid, ethanolamine and toluene; reacting at 100-120 DEG C for 4-7 hours; adding N-hydroxymethyl acrylamide and a polymerization inhibitor; and continuously reacting under the conditions that the vacuum degree is less than 0.1 MPa and the temperature is 50-70 DEG C to prepare the borate coupling agent. The borate coupling agent is polymerizable borate with an amino reaction active group, and has superior hydrolytic stability and good dissolubility. The borate coupling agent is a coupling agent with amino, alkenyl and hydroxyl active groups with boron atom as center, and has stronger interface effect. According to the invention, the boric acid reacts with the ethanolamine and the N-hydroxymethyl acrylamide; the toluene is used as a solvent; vapor produced by esterification is brought away through an azeotropic dehydration method; borate is gradually produced; the reverse reaction rate is reduced; and the product yield is improved.