Nanotheranostic platforms integrated with diagnostic and therapeutic functions have been widely developed for tumor medicine. However, the “always-on” nanotheranostic platforms suffer from poor tumor specificity, which may largely restrict therapeutic efficacy and prevent precise theranostics. Here, we develop an in situ transformable pro-nanotheranostic platform (ZnS/Cu2O@ZIF-8@PVP) by encapsulating ZnS and Cu2O nanoparticles in a metal-organic framework (MOF) nanomaterial of ZIF-8 that allows activable photoacoustic (PA) imaging and synergistic photothermal/chemodynamic therapy (PTT/CDT) of tumors in vivo. It is shown that the pro-nanotheranostic platform gradually decomposes and releases ZnS nanoparticles and Cu+ ions in acidic conditions, which spontaneously trigger a cation exchange reaction and synthesize Cu2S nanodots in situ with activated PA signals and PTT effects. Moreover, the excessive Cu+ ions function as Fenton-like catalysts and catalyze the production of highly reactive hydroxyl radicals (•OH) for CDT using elevated levels of H2O2 in tumor microenvironments (TMEs). In vivo studies demonstrate that the in situ transformable pro-nanotheranostic platform can specifically image tumors via PA and photothermal imaging and efficiently ablate tumors through synergistic CDT/PTT. Our in situ transformable pro-nanotheranostic platform could provide a new arsenal for precise theranostics in cancer therapy.