A pH-triggered drug delivery system that can specially target to folate-receptors-overexpressing cancer cells was designed based on newly developed polydopamine capsules and folic acid. The polycapsules bioconjugates possess capabilities of high drug loading, enhanced folate-receptor-mediated cell uptake, and sustained drug release by intracellular pH changes. It was also found that the sustained release of drugs significantly increased reactive oxygen species (ROS) level in targeted cells. The efficient cell-specific endocytosis and intracellular pH-responsive controlled drug delivery of the capsules is promising for future in vivo therapeutic applications.