Nickel catalyst was used to catalyze the hydrogenation of naphthalene in industry to obtain an equivalent mixture of two isomers. If pure isomers are needed, industrial decalin can be washed with concentrated sulfuric acid until the acid layer is colorless. Then washing with alkali and water, separating the water layer, and drying with desiccant. Vacuum distillation: in the column with theoretical plate number of 130, the reflux ratio is close to 145: 1, and the cis isomer with purity of 99.93 wt% can be obtained; In a column with 200 theoretical plates, the trans isomer with a purity of 99.97% can be obtained when the reflux ratio is 160: 1.