To obtain water-soluble oligodepsipeptide with pendant thiol groups, the alternating co-oligomer [oligo(Glc-alt-Cys)], consisting of glycolic acid (Glc) and L-cysteine (Cys) residues as alpha-hydroxy acid and alpha-amino acid residues, respectively, was prepared by means of ring-opening homo-oligomerization of cycle [Glc-Cys(MBzl)] and subsequent deprotection of methoxybenzyl groups. Moreover, to modify the properties of poly(lactic acid) [poly(LA)] and to introduce pendant thiol groups to poly(LA), the terpolymer of LA, Glc, and Cys {poly[LA-(Glc-Cys)]} was synthesized through ring-opening and copolymerization of L-lactide with the protected cyclodepsipeptide, cycle [Glc-Cys(MBzl)] and subsequent deprotection of methoxybenzyl groups. By changing the mel fraction of(Glc-Cys) unit, the solubility, thermal transition, degradation behavior of the modified poly(LA), and the water contact angle of its film could be varied.