We simulate the modal purities and Laguerre-Gaussian (LG) modal coupling of orbital-angular-momentum (OAM) beams reflected by a rough surface for a non-line-of-sight (NLoS) THz communication link. The reflection of the THz OAM beams is simulated using the finite-difference time-domain (FDTD) method. The simulation results show that: (i) an OAM beam reflected by a metal or dielectric rough surface would experience modal coupling in the 2-dimensional LG modal spectra; (ii) reflected by a metal surface, the modal purities of both transverse-electric (TE) and transverse-magnetic (TM) OAM beams increase as the incident angle increases from 0° to 70° or the carrier frequency of the OAM beams decreases from 1 THz to 0.1 THz; and (iii) reflected by a dielectric surface (n = 1.5), the modal purity and received power of the transmitted TE OAM beam also increase with an increase in the incident angle. However, for TM polarization, the modal purity exhibits a dip near the Brewster's angle. Moreover, with a limited-size receiver aperture, the simulation results for a rough metal surface show that: (i) when the aperture size decreases, the received power of the desired mode decreases by >30 dB while the modal purity of that mode firstly increases and then decreases; and (ii) with the same aperture size, the link could experience ~10 dB higher total power loss and ~12 dB higher channel crosstalk when the standard deviation of the surface height increases from 0.08 mm to 0.20 mm.