Nanoparticle- and microsphere-based drug delivery systems (DDSs) have attracted wide attention in cancer therapy; those DDSs that are responsive to tumor environment can selectively identify tumor and normal tissues and therefore have shown enhanced anticancer efficacy and alleviated systemic toxicity. Here, tumor-pH-sensitive polymeric microspheres, which are prepared by multiblock poly(l-lactide) with pH-sensitive acetal bonds in the backbone, are employed to efficiently load water-soluble anticancer drug doxorubicin hydrochloride (DOX·HCl, drug loading content: ∼10%). The pH-sensitive DOX-loaded hollow microspheres were in the size range 2-10 μm and exhibited acid-accelerated degradation of polymer matrix and drug release, and thereby efficient in vitro cancer cell inhibition. The microspheres were further intratumorally injected into breast-tumor-bearing mice, and the in vivo anticancer experiment showed that pH-sensitive DOX-loaded microsphere showed better antitumor efficiency and prolonged life-span than its counterpart that does not have pH-responsive property. Moreover, negligible organ toxicity, especially cardiotoxicity that generally exists in DOX-involved chemotherapy where DOX is administrated by intravenous injection, was observed for DOX-loaded microspheres. Hence, tumor-pH-sensitive polymeric microspheres have appeared to be a simple and efficient platform for delivering hydrophilic anticancer drug with excellent anticancer efficacy and low systemic toxicity.