Imaging guided photothermal agents have attracted great attention for accurate diagnosis and treatment of tumors. Herein, multifunctional NaYF4:Yb/Er@polypyrrole (PPy) core-shell nanoplates are developed by combining a thermal decomposition reaction and a chemical oxidative polymerization reaction. Within such a composite nanomaterial, the core of the NaYF4:Yb/Er nanoplate can serve as an efficient nanoprobe for upconversion luminescence (UCL)/X-ray computed tomography (CT) dual-modal imaging, the shell of the PPy shows strong near infrared (NIR) region absorption and makes it effective in photothermal ablation of cancer cells and infrared thermal imaging in vivo. Thus, this platform can be simultaneously used for cancer diagnosis and photothermal therapy, and compensates for the deficiencies of individual imaging modalities and satisfies the higher requirements on the efficiency and accuracy for diagnosis and therapy of cancer. The results further provide some insight into the exploration of multifunctional nanocomposites in the photothermal theragnosis therapy of cancers.