NOVELTY - Preparing Temsirolimus (RTM: Intravenous drug) comprises e.g. adding 3-hydroxy-2-hydroxymethyl-2-methyl-propionic acid and organic alkali into an organic solvent, adding chlorosilane compound into the reaction solution under ice-water bath, heating the reaction solution to room temperature, and reacting the materials to obtain substituted 2-methyl-3-silyloxy-2-(silyloxymethyl)propanoic acid (I), and dissolving (I) and the alkali in an organic solvent, and adding organic solvent.    USE - The method is useful for preparing Temsirolimus (RTM: Intravenous drug) (claimed), which is useful for treating cancer.    ADVANTAGE - The method simply and economically prepares the Temsirolimus (RTM: Intravenous drug) with short synthetic route and high yield.    DETAILED DESCRIPTION - Preparing Temsirolimus (RTM: Intravenous drug) comprises (1) adding 3-hydroxy-2-hydroxymethyl-2-methyl-propionic acid and organic alkali into an organic solvent, adding chlorosilane compound of formula (R1R2R3SiCl) (I) into the reaction solution under ice-water bath and then heating the reaction solution to room temperature, and reacting the materials to obtain substituted 2-methyl-3-silyloxy-2-(silyloxymethyl)propanoic acid compound of formula (II), (2) dissolving (II) and alkali in the organic solvent, adding organic solvent with substituted 1-((2R,3R)-2-hydroxy-3-methyl-tetrahydropyran-2-yl)-2-(1-piperidyl)ethane-1,2-dione compounds of formula (II) and 4-(N,N-dimethylamion) pyridine into the reaction solution and then reacting to obtain substituted 1-((2R,3R)-2-hydroxy-3-methyl-tetrahydropyran-2-yl)-2-(1-piperidyl)ethane-1,2-dione compounds of formula (III), and (3) reacting (IV) and an acid under solvent condition.    R1-R3 = H, 1-6C alkyl, benzyl or p-methylbenzyl independently, where R1-R3 are not H simultaneously;    P = H or protecting group, preferably acetyl or silane group of formula (SiR4R5R6); and    R4-R6 = H, 1-6C alkane, phenyl, benzyl or p-methoxybenzyl phenyl, benzyl or p-methoxybenzyl, where R4-R6 are not H simultaneously.