The invention discloses a preparation method of 1‑(4‑methoxy‑benzyl)‑2‑methyl‑octahydroisoquinoline. First, 1, 2, 3, 4, 5, 6, 7, 8 ‑Octahydro‑1‑(methoxyphenyl)methylisoquinoline is dissolved in the solvent, then add formaldehyde solution, after stirring, then add the catalyst, and feed hydrogen to carry out the catalytic hydrogenation reaction, after the reaction, filter out Catalyst, and separated by precipitation and distillation to obtain 1-(4-methoxy-benzyl)-2-methyl-1,2,3,4,5,6,7,8-octahydroisoquinoline . The method of the invention has the characteristics of mild and safe reaction conditions, green environmental protection, reduced side reactions, high yield, good purity, simple and convenient operation, etc., which enables the preparation industry to achieve green environmental protection, safety and high efficiency, and simultaneously reduces production costs.