A new propagation mechanism of guided gravity waves supported by the upper atmospheric wind is suggested in this paper. The numerical results show that,the existence of strong upper atmospheric winds make possible the long-distancemedium-scale guided gravity waves. The observed superior distributions of horizontal phase velocity at Millstone Hill are just correspond to those modes with longerattenuation distances. When the period of wave is smaller, the attenuation distanceof the guided wave strongly depends on the magnitude of wind speed along the antipropagation direction. The stronger the wind on the anti--propagation direction,the longer the attenuation distance, hence the more easily to be observed. Whenthe period becomes larger or the wind speed along the anti--propagation direction issmall, the ducting mechanism vanishes indeed. The above studies are in good accordance with the observation result at Millstone Hill, and give another interpretablepropagation mechanism of observed medium-scale gravity waves at mid--latitude.