The invention relates to a preparation method for a polyester fiber with composite ultraviolet resisting and cationic dyeing functions. The preparation method comprises an esterification reaction procedure, an interesterification reaction procedure, a polymerization reaction procedure and a spinning procedure, wherein three monomers are added in the interesterification reaction procedure; after esterification reaction and before pre-polymerization reaction, a hindered amine light stabilizer and a hindered phenol anti-oxidant are added; in the polymerization reaction procedure, after the pre-polymerization and before final polymerization, an ultraviolet absorbent is added; and before the spinning procedure, polyester obtained by the polymerization reaction procedure is sliced for pre-crystallization and drying. Fabrics prepared from the polyester fibers have good ultraviolet resisting effects, and can be dyed into various colors by cationic dyes of the polyester fiber at the same time,so the polyester fiber meets the requirements of conventional high-end spring and summer clothing on uniformity between functions and colors, has vast market prospect and is an important supplement to the conventional cationic-dyeable polyester fiber market in China.