Abstract: The propagation characteristics of electromagnetic waves in different shape roadway are different, and are also different in different modes. On the basis of analysis of theory of electromagnetic wave transmission and multi-mode theory, the influence of shape factor on electromagnetic waves propagation attenuation in rectangular roadway under high-order mode was researched by computer simulation. The simulation results show that when the shape factor of rectangular roadway is constant, the attenuation of electromagnetic wave increases with increase of electromagnetic wave transmission mode. Under different modes, with the increase of the shape factor, the slope of attenuation curve of electromagnetic wave remains unchanged. In the horizontal polarization mode, the attenuation of electromagnetic wave first decreases and then increase with increase of the shape factor, and the attenuation rate is the smallest when the shape factor is 1.7. In the vertical polarization mode, the attenuation curve also tends to decrease first and then increase with increase of the shape factor, and the attenuation rate is the smallest when the shape factor is 0.7. When the shape factor is equal to 1, the influence of the horizontal polarization and vertical polarization modes on electromagnetic wave propagation attenuation is same. When the shape factor is less than 1, the influence of the vertical polarization mode on the electromagnetic wave attenuation is greater than that of the horizontal polarization mode. And when the shape factor is greater than 1, influence of the horizontal polarization mode on electromagnetic wave propagation attenuation is greater than that of vertical polarization mode.