Abstract: Tent law and ray tracing method were used to simulate the multipath propagation characteristic of microwave in rectangular tunnel. Simulation results showed tha t, with the same roughness of a tunnel, both the delay spread and the signal att enuation varied with the carrier frequency. The higher the frequency, the smalle r the delay spread, but the larger the propagation attenuation. It was due to th e scattering effect. High frequency carrier, with short wavelength, will suffer larger scattering attenuation at each reflecting spot, therefore the number of m ultipath routes decreases. In addition, the delay spread of vertical polarized w ave is smaller than that of parallel polarized wave. It was due to the differenc e of reflecting coefficients of vertical polarized wave and parallel polarized w ave.