Dual stimuli-responsive camptothecin polymeric prodrugs (CPT Prodrugs) with grafted structures were designed via chemoenzymatic methods and combined with doxorubicin (DOX) for synergistic drug delivery to improve anticancer efficiency. The CPT Prodrugs loaded DOX with a high efficiency through the cooperative contribution of several interaction forces. The produced amphiphilic polymeric prodrugs loaded with DOX, referred to as DOX@CPT Prodrugs, formed homogeneous spherical micelles of appropriate sizes (sub-50 nm). The DOX@CPT Prodrug micelles showed excellent stability in release experiments under in vitro physiological conditions and maintained over 80% drug loading after 4 weeks when stored at 4 °C. Under weakly acidic pH and reduced glutathione (GSH) conditions, the DOX@CPT Prodrugs with high disulfide and tertiary amine content achieved synergistic release of the two loaded drugs and biodegraded into low-molecular-weight compounds. The cell experiments confirmed that the internalization of DOX@CPT Prodrugs into cancer cells was greatly improved by nearly 30% compared with that of free drugs. Furthermore, the synergistic drug delivery system exhibited superior anticancer efficiency with highly improved cell selectivity ratios (up to 127.0%) and greatly enhanced synergistic effects (up to 23.9%) benefiting from good long-term stability, better internalization by cells and sensitive pH and GSH dual-responsivity. In addition, the DOX@CPT Prodrugs with suitable sizes and good water solubility also exhibited a greater penetrability in the case of simulating solid tumors than the free drugs. These results demonstrate the potential of DOX@CPT Prodrugs as biodegradable, dual-responsive combination therapy nanocarriers for synergistic anticancer treatment.