A synthetic method of 20(s)-ginsenoside Rh2, that is 20(S)-protopanaxdiol-3-O-.beta.-D-glucopyranoside, is comprised of: protecting protopanaxdiol (A1) selectively first to produce monosubstituted protopanaxdiol (A2); and Glycosidating the monosubstituted protopanaxdiol with Glucopyranosyl donor in the presence of Lewis acid catalyst; Deprotecting the product; Then separating and purifying to obtain 20(s)-ginsenoside Rh2. The method is conducted under mild condition at low cost, and affords product with high stereoselectivity, high yield and purity. Therefore, the synthetic method of the present invention is suitable for production on large scale.