The invention discloses a novel preparation method for an anti-type-II-diabetes drug sitagliptin. According to the invention, trifluorobenzene with a cheaper price is used as a fluorization reagent and a starting raw material and a basic skeleton of sitagliptin is successfully synthesized through effective acylation of trifluorobenzene and L-aspartic acid; a synthesis route in the invention, from starting raw material and final product, is completely different from schemes disclosed in the prior art; the route is optimized, and easily-available natural L-aspartic acid is used as a chiral source for successful synthesis of an optically pure sitagliptin product; the problems of asymmetric catalysis and complex splitting in the prior art are overcome; the method also effectively overcomes the problem of low yield of the basic skeleton of sitagliptin synthesized via Friedel-Crafts acylation of trifluorobenzene and improves yield; the method is lower in cost, more convenient to operate and more suitable for industrial production; and compared with the prior art, the method is simpler in the synthesis route and better in operability.