Magnetic nanoparticles as well as core-shell magnetic nanocomposites are of great interest for researchers due to their potential applications in lots of areas. In the present review, the authors summarized several universal synthetic methods of nanocomposites and their specific properties. In the following, the authors focused on the applications of functionalized magnetic nanoparticles in separation and spectral detection, along with the introduction of some work in the authors’ lab. At last, the questions remaining in magnetic nanoparticles and the application perspectives of magnetic nanocomposites were discussed.