Accurate diagnosis of tumor is promising to guide photothermal therapy (PTT) for efficacious tumor ablation with minimal damage to healthy tissues. Here, we report an activatable dual-modal imaging agent, which is based on PEGylated-gadolinium metallofullerenepolypyrrole nanoparticle (PEG-GMF-PPy NP) for imaging-guided PTT. A contrast agent (gadolinium metallofullerene, GMF) with excellent magnetic resonance imaging (MM) performance and an ultra-pH-responsive polymer (PEG-PC7A) are successively modified to the surface of photothermal agent (PPy NP). The prepared PEG-GMF-PPy NPs show strong absorption in the near-infrared (NIR) region, so they can be utilized for photoacoustic imaging. Furthermore, in a tumor extracellular environment, the PEG-GMF-PPy NPs can achieve pH-enhanced MRI because of the hydrophobic-to-hydrophilic conversion of the PC7A. Upon accurate diagnosis-guided NIR laser irradiation, excellent tumor ablation effect is achieved. The results suggest that the PEG-GMF-PPy NPs are promising agents for activatable imaging-guided PTT.