Targeted photodynamic therapy (TPDT) based on the photosensitizers responsive for tumor microenvironment is promising because of the better anti-tumor effect and less phototoxicity against normal tissue than the traditional PDT. Nanoparticle-based stimuli-responsive photosensitizers have been widely explored for TPDT. Based on the acidic microenvironments in solid tumors, an ultrasmall pH-responsive silicon phthalocyanine nanomicelle (PSN) (smaller than 10$$nm) was designed for selective PDT of tumor. PSN had high drug loading efficacy (more than 28%) and exhibited morphological transitions, enhanced fluorescence and improved singlet oxygen yield under acidic environments. PSN was renal clearable and could rapidly accumulate and be retained at tumor sites, achieving a tumor-inhibiting effect better than phthalocyanine micelle without pH response. Tumors of mice treated with PSN for PDT were completely ablated without recurrence. Thus, we have developed a phthalocyanine-based pH-responsive micelle with excellent tumor targeting ability, which is expected to realize the selective PDT of tumor.Targeted photodynamic therapy (TPDT) based on the photosensitizers responsive for tumor microenvironment is promising because of the better anti-tumor effect and less phototoxicity against normal tissue than the traditional PDT. Nanoparticle-based stimuli-responsive photosensitizers have been widely explored for TPDT. Based on the acidic microenvironments in solid tumors, an ultrasmall pH-responsive silicon phthalocyanine nanomicelle (PSN) (smaller than 10$$nm) was designed for selective PDT of tumor. PSN had high drug loading efficacy (more than 28%) and exhibited morphological transitions, enhanced fluorescence and improved singlet oxygen yield under acidic environments. PSN was renal clearable and could rapidly accumulate and be retained at tumor sites, achieving a tumor-inhibiting effect better than phthalocyanine micelle without pH response. Tumors of mice treated with PSN for PDT were completely ablated without recurrence. Thus, we have developed a phthalocyanine-based pH-responsive micelle with excellent tumor targeting ability, which is expected to realize the selective PDT of tumor.