The invention discloses a method for preparing a rosuvastatin calcium midbody, namely a compound (R is C1-C10 alkyl, and Y is a hydroxyl protecting group) shown as the general formula I. Chloroethylene and R-epoxy chloropropane as initial raw materials are carried out seven steps of reaction, such as Grignard reaction, sodium cyanide nucleophilic substitution reaction, alcoholysis reaction, hydroxyl protection, oxidizing reaction, methylchloroformate acylation reaction and Wittig reaction to prepare the compound shown as the general formula I. The method has mild condition, simple and convenient operation, stable process, low cost and easy acquisition of raw materials, high product yield, easy disposal of the three wastes, less environmental pollution, low preparation cost and suitability for industrialized large-scale production.