The present invention provides a process for the preparation of (6-per-deoxy-6-per-(2-carboxyethyl)thio-³-cyclodextrin) sodium salt, comprising the steps of: reacting ³-cyclodextrin (SM1) with iodine in the presence of triphenylphosphine in an organic solvent to afford an intermediate, 6-per-deoxy-6-iodo-³-cyclodextrin (abbreviated as SGMD-1)； adding methanol solution of sodium methoxide into the reaction system followed by the addition of acetone without removal of the solvent under reduced pressure to obtain the crude SGMD-1 as a solid after filtration； purifying the crude SGMD-1 by recrystallization； reacting a obtained recrystallized intermediate (SGMD-1) with 3-mercaptopropionic acid in basic medium e.g., sodium hydride, to obtain the crude 6-per-deoxy -6-per-(2-carboxyethyl)thio-³-cyclodextrin sodium salt (abbreviated as SGMD)； purifying the crude SGMD by passing through adsorbents followed by recrystallization.
