Photothermal therapy (PTT) is a powerful technique for tumor ablation. However, there is a problem that PTT cannot accurately locate the tumor site, so it is easy to cause heat damage to the surrounding normal tissues. In this study, PEGylated amorphous manganese dioxide (MnO₂) coated polydopamine (PDA) core-shell nanoparticles (PDA@MnO₂-PEG) with regular morphology and uniform dimensions were prepared for acid-sensitive magnetic resonance imaging-guided tumor photothermal therapy. The results showed that amorphous MnO₂ shell provided markedly acid-sensitive T1-weighted magnetic resonance imaging (MRI). The relaxation rate (r1 value) of PDA@MnO₂-PEG in pH=7.4 was measured to be 0.310 mM-1·S-1, while that in pH=6 became 4.364 mM-1·S-1. In mice tumor models, MRI of tumors exhibited dramatically whitening effects compared with the signal before injection. Besides, the in vivo experiments revealed that the tumors in PTT group with PDA@MnO₂-PEG injection and NIR laser irradiation were almost eliminated within 14 days, indicating the effective photothermal therapy of tumor generated by the PDA core. In conclusion, we successfully synthesized amorphous MnO₂ coated PDA core-shell nanoparticles with the function of acid-sensitive magnetic resonance imaging guided photothermal therapy, which provide a new approach for improving the effect of photothermal therapy of tumors.