Terahertz (THz) transmission simulations play an important role in THz technology researches, especially for the structural design of a THz waveguide. Ray model takes into account both structure parameter of waveguide and the divergence angle of beam light and could be an alternative way for THz transmission behavior simulations. In this paper, the ray model is used to calculate the transmission loss of tube waveguide, and the simulated transmission losses are presented to compare with the results calculated by COMSOL. The suitable THz frequency range of ray model is discussed by analyzing the transmission loss spectra of tube waveguides with various core sizes. The credibility of ray model on terahertz transmission simulations is demonstrated based on the experimental results tested by THz-TDS and calculated results.