The synthesis of polymer-drug conjugate (PDC) capable of convenient preparation and controlled release of therapeutic agents is still an urgent requirement in drug delivery field. Herein, we develop a novel anti-cancer PDC engineered with side groups of disulfide and ester bonds for on-demand delivery of paclitaxel (PTX) with redox and pH dual sensitive behaviors. A simple polymer, 3,3'-dithiodipropionic acid functionalized poly(ethylene glycol)-b-poly(l-lysine) (mPEG-b-P(LL-DTPA)), was synthesized and PTX was directly conjugated to the carboxyl groups of mPEG-b-P(LL-DTPA) to obtain the disulfide-containing polymer-PTX conjugate (P(L-SS-PTX)). Another structural similar polymer-PTX conjugate without disulfide bonds (P(L-PTX)) was also prepared to verify the function of disulfide linkages. The P(L-SS-PTX) micelles showed rapid drug release under tumor-relevant reductive conditions as designed. Interestingly, the PTX release from P(L-SS-PTX) micelles could also be promoted by the increased acidity (pH ≈ 5). In vitro cytotoxicity study showed that the P(L-SS-PTX) micelles exhibited significantly enhanced cytotoxicity against a variety of tumor cells compared to the non-sensitive P(L-PTX) micelles. The in vivo studies on B16F1 melanoma bearing C57BL/6 mice demonstrated the superior antitumor activity of P(L-SS-PTX) over both free PTX and P(L-PTX). This dual-sensitive prodrug provides a useful strategy for anti-tumor drug delivery.