Progress in photoacoustic (PA) and magnetic resonance imaging (MRI) bimodal contrast agents has been achieved mainly by utilizing the imaging capability of single or multiple components and consequently realizing the desired application for both imaging modalities. However, the mechanism of the mutual influence between components within a single nanoformulation, which is the key to developing high-performance multimodal contrast agents, has yet to be fully understood. Herein, by integrating conjugated polymers (CPs) with iron oxide (IO) nanoparticles using an amphiphilic polymer, a bimodal contrast agent named CP-IO is developed, displaying 45% amplified PA signal intensity as compared to bare CP nanoparticle, while the performance of MRI is not affected. Further experimental and theoretical simulation results reveal that the addition of IO nanoparticles in CP-IO nanocomposites contributes to this PA signal amplification through a synergistic effect of additional heat generation and faster heat dissipation. Besides, the feasibility of CP-IO nanocomposites acting as PA-MRI bimodal contrast agents is validated through in vivo tumor imaging using mice models. From this study, it is demonstrated that a delicately designed structural arrangement of various components in a contrast agent could potentially lead to a superior performance in the imaging capability.