Abstract: The terahertz (THz) band lies between the microwave and infrared regions of the electromagnetic spectrum and has important applications in security screening, biomedical imaging, and industrial nondestructive testing. In this paper, cesium atoms are taken as the research object, and theoretical modeling and numerical simulations are carried out to investigate Rydberg-atom-based THz transitions and imaging mechanisms. A Monte Carlo fluorescence decay model incorporating blackbody-radiation-induced transitions and atomic collision effects is developed to simulate the fluorescence spectral characteristics and spontaneous decay pathways of the cesium n=13 Rydberg state, revealing the dominant radiative transition channels. On this basis, a transition-channel selection criterion suitable for THz imaging is established by combining fluorescence contrast with the dipole matrix elements of THz transitions, and multiple efficient transition channels are identified within the 500~2 000 GHz frequency range. Furthermore, the imaging performances of two-photon excitation (with an intermediate state of 6P_3/2) and three-photon excitation (with an intermediate state of 7S_1/2) schemes are comparatively analyzed. The results demonstrate that the two-photon scheme exhibits clear advantages in reducing system complexity, expanding the number of available transitions, and enhancing fluorescence signal contrast. These simulation results provide a theoretical foundation for the experimental design and performance optimization of Rydberg-atom-based THz imaging systems.