A Pt prodrug polyphenol and gadolinium ion loaded cancer theranostics nanoplatform based on a mild acidic pH and thermal sensitive polymer was designed for photoacoustic (PA)/magnetic resonance (MR)/positron emission tomography (PET) multimodal imaging-guided chemo-photothermal combination therapy. The Pt drug release can be controlled by tumour-specific acidic pH and heat generated by external NIR irradiation. The nanoparticles were stable under normal physiological environments and released the drug under the acidic pH of tumours and NIR laser irradiation, which can reduce the side effects of the drug to normal organs. Moreover, the MR signal can be significantly enhanced (B3-fold increase in T1 relaxivity) under the acidic tumour microenvironment, which is favorable for cancer diagnosis. The nanoparticles exhibited excellent tumour accumulation and led to complete tumour eradication with low power NIR laser irradiation. This promising approach provides a new avenue for imaging-guided combination therapy.