  Terahertz (THz) channel propagation characteristics are vital for the design,
evaluation, and optimization for THz communication systems. Moreover,
reflection plays a significant role in channel propagation. In this letter, the
reflection coefficient of the THz channel is researched based on extensive
measurement campaigns. Firstly, we set up the THz channel sounder from 220 to
320 GHz with the incident angle ranging from 10{\deg} to 80{\deg}. Based on the
measured propagation loss, the reflection coefficients of five building
materials, i.e., glass, tile, aluminium alloy, board, and plasterboard, are
calculated separately for frequencies and incident angles. It is found that the
lack of THz relative parameters leads to the Fresnel model of non-metallic
materials can not fit the measured data well. Thus, we propose a
frequency-angle two-dimensional reflection coefficient model by modifying the
Fresnel model with the Lorenz and Drude model. The proposed model characterizes
the frequency and incident angle for reflection coefficients and shows low
root-mean-square error with the measured data. Generally, these results are
useful for modeling THz channels.
