Overtreatment as a crucial modern medicine issue needs to be urgently addressed. Theranostic agents supply a unique platform and integrate multiple diagnosis and therapies to deal with this issue. In this study, a core-shell MnS@Bi2S3 nanostructure was fabricated via two step reactions for tri-modal imaging guided thermo-radio synergistic therapy. The mass ratio between the core and shell of the constructed MnS@Bi2S3 can be precisely controlled via cation exchange reaction. After surface PEGylation, MnS@Bi2S3-PEG nanoparticles exhibited excellent aqueous medium dispersibility for bioapplications. Based on the r1 and r2 relaxivity obtained from the MnS core and the strong near-infrared absorption and X-ray attenuation abilities of the Bi2S3 shell, the intratumoral injected MnS@Bi2S3-PEG can realize in vivo magnetic resonance, computer tomography, and photoacoustic tumor imaging under a single injection dose. Hyperthermia significantly boosts the efficacy of radiation therapy, showing synergistic tumor treatment efficacy. No obvious toxicity is monitored for the treated mice. Our study not only provides a new way to precisely construct the core-shell nanocomposite, but also presents a unique theranostic platform and unifies the solutions for the challenges related with high injection dose and overtreatment.