Stimuli-sensitive drug delivery systems have attracted considerable interest in recent years for biomaterials scientists. It is well known that the pH value drops from physiologically 7.4 to that lower than 6.5 in tumor tissues, and such difference can be used as a trigger for drug releasing. Poly(amino acid)s (PAAs) have been studied for decades in the fields of drug delivery system due to their biocompatibility, biodegradability, precise secondary conformation and structural versatility. In this study, a novel biodegradable ABC type triblock copolymer mPEG-poly(benzyl L-aspartate)-poly(L-Alanine) (mPEG-PBLA-PLAla) was synthesized by N-carboxyl anhydride ring-opening polymerization. Imidazole groups were tethered to the side chains of poly(L-Asparagine) segments by aminolysis. It was found that this tri-block copolymer can form nano-scale core-shell-corona trilayer micelles in aqueous solution. The PLAla, Poly(L-Asparagine) and mPEG segments serve as a hydrophobic core, a pH-sensitive shell, and a hydrophilic corona, respectively. An antitumor agent, doxorubicin (DOX), was successfully loaded into the nanocarrier via combined actions of hydrophobic and π-π interaction. The drug release profiles displayed a pH-dependent behavior. DOX release rate increased significantly as the solution pH dropped from the physiological pH to acidic. This is most likely due to protonation and a change in hydrophilicity of the imidazole groups in the poly(L-Asparagine).