With the development of 6G networks, in-depth research on the propagation characteristics of terahertz(THz) channels in urban environments is crucial for designing efficient, reliable, and secure communication systems. The impact of different types of building corners(including acute, right, obtuse, and curved angles) on the transmission of THz channels and their physical layer security are systematically investigated through a combination of theoretical analysis, numerical simulation, and experimental measurement. The experiments are conducted by using a THz channel measurement system at three frequencies: 140 GHz, 225 GHz, and 320 GHz. Theoretical analysis is performed by using numerical simulations and knife-edge diffraction models. The research findings reveal the effects of corner structures on the propagation of THz waves, including diffraction and reflection phenomena, as well as the impact of frequency variations on these phenomena. This work provides theoretical guidance for the deployment of THz communication systems in urban environments.