The propagation of light is generally reciprocal. Optical reciprocity, also known as temporal inversion symmetry, is essentially the existence of reciprocal solutions supported by Maxwell's equations. However, traditional reciprocal optical components suffer from problems such as the optical diffraction limit, which limits their further development toward high performance. In recent years, with the rapid development of topological photonics, it has been found that topological photon states have one-way transmission properties with forward conduction and reverse blocking state. At the same time, local structural defects will not influence global properties in optical topology, therefore, non-reciprocal topological photon states have many excellent physical properties, such as immunity to obstacles and defects, which can ensure that the one-way transmission has strong robustness and consequently promises potential applications such as optical integrated circuits and nonlinear optics. Magneto-optical photonic crystals based on gyromagnetic materials are the earliest topological optical structures to realize non-reciprocal topological photon states. They are also the most commonly used topological optical structures to study the generation, interaction and novel topological optical phenomena of topological photon states, which still contain a series of physical mechanisms worthy of further exploration. However, an external magnetic field is required, which affects the integration of optical devices, the frequency is limited to the microwave, and the gyromagnetic material has a weak response to the magnetic field, so it is still difficult to realize the integer quantum Hall effect of light, thus limiting its practical application prospects.Magnetoplasma is a simple material with non-reciprocal, rotational and homogeneous medium. The maximum non-reciprocity can be achieved by optimizing the permittivity tensor or the permeability tensor since the strength of the non-reciprocity depends on the relative strength of the diagonal tensor elements and the off-diagonal tensor elements. According to the Onsanger-Casimir principle, the dielectric constant tensor of a magnetic plasma is asymmetric, which implies the breakdown of the Lorentz reciprocity. The near-field electromagnetic wave propagating at the interface of magneto-optical materials under the action of a magnetic field is called a magnetic surface plasmon, which has non-reciprocal transmission characteristics. The unidirectional waveguide structure based on magnetic surface plasmon not only has a local enhancement effect, but also can break the diffraction limit, which lays a solid foundation for the development of new optical functional devices with high performance, high resolution, and high integration in space environment detection. At the same time, we also note that non-reciprocal transmission based on plasmon may be subject to absorption interference, and its practical value needs further verification. The optical nonlinear effect is also one of the ways to achieve non-reciprocal transmission. To achieve non-reciprocal transmission, the most likely development direction is to design topologically protected nonlinear optical phenomena in accordance with the topological edge soliton phase formed by Hall edge states in photonic crystals. It is easy to realize the system integration without the external magnetic field to achieve non-reciprocal transmission. However, in order to achieve nonlinear effects, it usually requires extremely high electric/magnetic field strength or extensive use of nonlinear materials, which greatly limits its application. Finally, this paper also briefly discusses non-reciprocal transmission based on other principles, such as breaking optical reciprocity using time modulation, non-reciprocal optical transmission based on synthetic angular momentum, and the use of coupling characteristics between nonlinear resonators, and so on. Non-reciprocal optical transmission based on time, phase, and energy modulation, as well as losses, also faces instability issues. This is currently a major research direction in the field, with one potential solution being to design systems related to and protected by topology to realize non-reciprocal transmission, thereby reducing losses caused by reverse transport in optical devices. Additionally, we analyzed and looked forward to its future development trends and key issues it may face.