A method for preparing a radiopharmaceutical and, specifically, a method for preparing an organic fluoride-aliphatic compound usable as a radiopharmaceutical, a method for purifying the prepared organic fluoride-aliphatic compound, and a method for preparing a radiopharmaceutical by using a cassette comprising a backdraft preventing reaction container. A method for preparing an organic fluorinated aliphatic compound includes allowing a fluorine salt to react with a leaving group-containing aliphatic compound by using a multifunctional solvent represented by the following Chemical Formula 1 to obtain an aliphatic compound labeled with [18F] fluoride substituting for the leaving group. The organic fluoride-aliphatic compound can be prepared and purified through even a simple process at high yield, high efficiency, and high purity, and the radiopharmaceutical can be safely prepared without damage to a synthetic apparatus.