Through-wall radar (TWR) utilizes electromagnetic waves to penetrate non-metallic walls. It can not only obtain information of the layout of building interior walls and target positions in non line of sight situations, but also obtain the information of stationary life forms, trajectories of human bodies inside builidings, etc. The TWR could effectively provide useful information of the perception and judgment of complex urban battlefield situations. In this article, a brief information to the through-wall radar is first provided. Then, this study analyzes the principles of commonly used TWR such as synthetic aperture radar and multi receiver and multi transmitter radar. In addition, typical imaging methods including imaging methods based frequency domain, imaging methods based time domain and compression sensing imaging method are provided. What?s more, typical applications and other key technologies of TWR are systematically summarized. Finally, the future research directions of the through wall radar is analyzed. This article has guiding significance for the development of TWR.