Abstract: The traditional Gaussian beam tracing and predicting model doesn’t take a special treatment of diffraction effects, which are accounted for by superposition of transformed beam fields. A new 3D Gaussian beam-tracing and predicting model in complex metropolis environment is proposed. The beam tracing algorithm is based on geometrical optics and the uniform geometrical theory of diffraction. Diffraction effects are evaluated by using complex ray formulas and a new heuristic uniform diffraction coefficient for nonperfectly conducting wedges. The effects of propagation distance and order of reflection on predicting precision and complexity are also analyzed. The simulation results show that the precision of our new method is 0.02 to 2.2 dB higher than old Gaussian beam-tracing method and meanwhile the calculation efficiency is decreased by 7.5 to 10 percent in Ottawa. And it is found that 12 is a good value for the order of reflection considering reflection loss and beam transverse extension.