A method for synthesis of 1-decene oligomer is provided, wherein 1-decene is polymerized at 80-120° C., 0.8-1.4 MPa in the presence of aluminum trichloride catalyst supported on gamma-alumina and n-hexane solvent where the volume ratio of 1-decene to n-hexane is 3:8-4:1. The catalyst is treated as follows: impregnating gamma-alumina carrier in 0.5-2.0 M of hydrochloric acid, sulfuric acid, nitric acid or mixtures thereof, then vacuum drying at 80-100° C. and calcining at 400-800° C.; dissolving 5-10 g of anhydrous aluminum trichloride in 100 ml of tetrachloromethane, trichloromethane or dichloromethane solvent; adding the obtained solution into 10-20 g of activated alumina carrier and obtaining the catalyst after vacuum drying. The conversion of 1-decene is 50 wt % or more. The oligomer has a kinematic viscosity at 40° C. of 6.0-25 mm2/s and a viscosity index of 160-262.