We designed a pH-responsive fluorescent carbon nanoparticle for tumor detection and killing by loading a photothermal dye IR825 into the carbonized zwitterionic polymer [FNP-I]. The fluorescence of FNP-I, which was “switch-off” at pH 7.4, was activated (“switch-on”) when entering the cancer cell and release IR-825 owing to the alteration of zwitterionic FNP hydrophobicity. Moreover, the released of IR-825 generated high photothermal effect under NIR irradiation and killed the cancer cell. When applied FNP-I in mice bearing MDA-MB-231 cell-derived tumor, same mechanism of fluorescent “switch-on” and IR825 release was observed, resulting signal for tumor detection and photothermal therapy for tumor killing. Thus, this system provides excellent potency for tumor imaging and treatment based on photothermal effect both in vitro and in vivo.