The invention discloses a sargassum thunbergii lectin and a preparation method thereof, wherein the preparation method of the sargassum thunbergii lectin is characterized by selecting sargassum thunbergii as a raw material, carrying out lyophilization, pulverization, immersion, centrifugation, ammonium sulfate fractionation, DEAE-52 ion-exchange chromatography and SephadexG-200 gel filtration chromatography, thus producing the sargassum thunbergii lectin with a molecular weight of 17 kD. The sargassum thunbergii lectin of the invention can agglutinate a wide range of cells, has agglutinative functions for erythrocytes of rabbits, dogs, sheep, crucians, chickens and humans (A, B, AB), and the like, wherein minimum concentrations of agglutination reactions with rabbit and chicken erythrocytes are 4.4mg / ml and 0.55 mg / ml respectively, and shows hemagglutinin activity for rabbit erythrocytes even after heating treatment at 100 DEG C for 30 min (the activity decreases by 75%). A sugar inhibition experiment shows that the sargassum thunbergii lectin is only inhibited by glycoproteins as bovine thyroglobulin and gamma-globulin, and the minimum inhibition concentrations are 1.25 mg.mL<-1> and 2.5 mg.mL<-1> respectively.