The development of multifunctional nanoparticles for tumor theranostics has become a research hotspot. Despite the advantages of non-invasive precision diagnostics and efficient drug-delivery, these nanoparticles bring two significant issues: (i) a potential toxic effect and (ii) difficult clearance. To solve these issues, carbon dots (C-dots) are key potential candidates owing to their unique properties, such as excellent biocompatibility and rapid renal clearance. However, their small size leads to a short circulation time in the blood, which causes non-sufficient tumor accumulation for antitumor therapy. To reach the balance between an efficient accumulation in a tumor and rapid clearance from the body, herein we report a new multifunctional nanoprobe: photosensitizer (chlorine e6, Ce6)-loaded assembled C-dots (A-C-dots@Ce6). The A-C-dots@Ce6 were assembled from negatively-charged discrete C-dots using Gd3+ ions as a "glue". which also provided another function of in vivo nanoprobe monitoring via magnetic resonance (MR) imaging. Moreover, the nanoprobe exhibited an acidic pH-dependent disassembly and drug-release property. Benefiting from these advantages, the nanoprobe showed a targeted antitumor effect in A549 tumor-bearing mice under laser irradiation and gradual disassembly in the tumor for later body clearance. Therefore, the nanoprobe potentially provides a new strategy to solve the above balance issue, and brightens the future for antitumor monitoring and treatment.