The present invention relates to a method for preparing t-butyl 2-((4R,6S)-6-formyl-2,2-dimethyl-1,3-dioxan-4-yl)acetate having optical activity, which is a core intermediate in the preparation of a variety of HMG-CoA reductase inhibitors. According to the preparation method of the present invention, it is possible to easily mass-produce t-butyl 2-((4R,6S)-6-formyl-2,2-dimethyl-1,3-dioxan-4-yl)acetate in the form of a solid of high and uniform purity by using a polymer-supported TEMPO.