The invention discloses a preparation method of tadalafil. The concrete preparation method is used for successfully synthesizing tadalafil by taking L-tryptophan methyl ester hydrochloride as an initial raw material and utilizing the characteristics that an ortho-position of an ester group has the chiral inversion property under an alkaline condition, the reaction between ester group and Pictet-Spengler is reversible reaction, and the ester group is easily converted into a cis-form product with relatively small solubility, therefore, the preparation method is a brand new technology. The price of L-tryptophan methyl ester hydrochloride is only less than 1/5 of that of D-tryptophan methyl ester hydrochloride, so that the cost of the overall route is greatly lower than that of the prior art, and then the preparation method is suitable for industrial production.