Method for preparing tadalafil

The invention discloses a method for preparing tadalafil. The method comprises the steps that water and catalyst tetrabutyl ammonium bromide are added to a compound B, a methylamine water solution is added to the compound B, and the temperature of the mixed solution is increased to the range from 60 DEG C to 90 DEG C to be kept for 6-10 hours for reaction; after reaction, the mixed solution is cooled to the temperature ranging from 20 DEG C to 35 DEG C, the temperature is kept, devitrifying is carried out for 3-6 hours, filtering is carried out, and filter cakes are washed and dried to obtain finished tadalafil. Compared with the prior art, the price of the adopted methylamine water solution is lower than that of a methylamine methylamine solution, water is adopted as reaction solvent, production cost is greatly reduced, and water is safe and friendly to the environment. The purity of the tadalafil prepared through the method is more than 99.90 percent, and recrystallization process is not needed. The method is easy to operate, low in production cost and pollution and easier to produce in an industrial mode.