Terahertz (0.1-10 THz) band communication is envisioned as a key technology of future wireless communication because of its technical characteristics, such as super large bandwidth resources and super high communication rates. Herein, ray-tracing techniques and Beckman-Kirchhoff scattering theory are used to modify a unified multi-ray channel model in the terahertz band. The model combines direct, reflection, and scattering, and is supported by experimental data from the literatures. Furthermore, the wideband channel capacity usage is characterized using equal power and water-filling power allocation strategies. The simulation results show that the importance of resource allocation in exploiting the terahertz spectrum due to its extremely high frequency-selectivity. The study will provide some guidance for future research and design of the terahertz communication systems.