The integration of diagnostic and therapeutic functionalities on a single theranostic nano-system holds great promise to enhance the accuracy of diagnosis and improve the efficacy of therapy. Herein, a multifunctional polymeric nano-micelle system that contains a photosensitizer chlorin e6 (Ce6) is successfully fabricated, at the same time serving as a chelating agent for Gd3+, together with a near-infrared (NIR) dye, IR825. With a r(1) relativity 7 times higher than that of the commercial agent Magnevist, strong fluorescence offered by Ce6, and high NIR absorbance attributed to IR825, these theranostic micelles can be utilized as a contrast agent for triple modal magnetic resonance (MR), fluorescence, and photoacoustic imaging of tumors in a mouse model. The combined photothermal and photodynamic therapy is then carried out, achieving a synergistic anti-tumor effect both in vitro and in vivo. Different from single photo treatment modalities which only affect the superficial region of the tumor under mild doses, the combination therapy at the same dose using this agent is able to induce significant damage to both superficial and deep parts of the tumor. Therefore, this work presents a polymer based theranostic platform with great potential in multimodal imaging and combination therapy of cancer.