The invention discloses a method for preparing sofosbuvir and belongs to the technical field of medicine synthesis. The method comprises the following steps: putting tertiary butyl magnesium halide into a tetrahydrofuran solution of a compound 2 with certain amount of water, further putting a compound 3, and performing reaction, thereby obtaining sofosbuvir. As a certain amount of water is added in a reaction system, the conversion rate of reaction is remarkably increased, the content of main byproducts is reduced, then the preparation cost of the sofosbuvir is lowered, and remarkable social and economic benefits are achieved.