According to Lorentz reciprocity theorem, we know that in ordinary optical systems, the received optical signal is unchanged even if the positions of the signal source and the detector are exchanged. This brings us convenience for optical design and analysis. However, in all-optical communication and quantum network, it is necessary to effectively control the directional transmission of optical signals to avoid the interference of reflected light and separate the optical signals transmitted in opposite direction. Therefore, nonreciprocal devices that only allow one-way transmission of light are also essential components. This review starts from the background of the magneto-optical isolators and then briefly introduces the research status of the magneto-optical field. The research focus is the development of magnetic-free nonreciprocity, from both the active and passive aspects, which undoubtedly demonstrates the important role of quantum optics in the field of magnetic-free nonreciprocity.