Abstract: Magnetic particle imaging (MPI) is a new imaging technique with high sensitivity, high temporal resolution and spatial resolution, which allows to detect real-time 3D spatial distribution of magnetic nanoparticles (MNPs). Color MPI allows to image not only the spatial distribution of magnetic nanoparticle concentration, but also the spatial distribution of in-situ temperature and biomolecule concentration, which has shown great promise in nanomedicine-based precision medicine, including cancer diagnostics and therapy (theranostics). Current MPI approaches are faced with two main challenges, including unclear underlying physical mechanism of nanomagnetism and signal-to-noise bottleneck of precise measuring instruments. Aiming at the current MPI technique, the MPI development and its applications in biomedicine such as cancer theranostics are reviewed.