A pH and thermo dual‐responsive supramolecular diblock copolymer is constructed by host–guest recognition of pillar[5]arene and viologen salt. The host polymer, poly(N,N‐dimethylaminoethyl methacrylate) bearing pillar[5]arene as the terminal group (P[5]A‐PDMAEMA) is synthesized by atom transfer radical polymerization (ATRP). Guest polymer, ethyl viologen‐ended poly(N‐isopropylacrylamide) (EV‐PNIPAM) is prepared by reversible addition–fragmentation chain transfer polymerization. The supramolecular diblock copolymer can be self‐assembled into stable supramolecular nanoparticles in aqueous solution at 40 °C, which show excellent pH and thermo responsiveness. The nanoparticles are further applied in the encapsulation of photosensitizers (pyropheophorbide‐a, PhA) for photodynamic therapy (PDT). The dual‐responsive nanoparticles can efficiently release PhA in acidic environment at 25 °C. Based on the result of cell experiments, PhA‐loaded nanomicelles exhibit excellent PDT efficacy and low dark toxicity toward A549 cells. Thus, this supramolecular diblock copolymer enriches the methodology of constructing stimuli‐responsive drug carriers and presents a great potential in PDT.