Abstract: This study uses ground station observations, ERA5 reanalysis datas and a high-resolution WRF model simulation to analyze a downslope windstorm event in Boulder, United States. It is believed that the mechanism of this downslope windstorm outbreak is the downward transfer of momentum to the foot of the leeward slope caused by the rupture of the high-altitude jet belt. Further analysis shows that the outbreak of the downslope windstorm is related to the passage of synoptic scale high-pressure ridges and local leeward slope gravity waves. Under the influence of the foehn, continuous warming on the leeward slope causes gravity waves to strengthen and turn vertically, eroding the top of the troposphere's jet zone and ultimately leading to the rupture of the jet with high-altitude momentum transferred to the ground, as a result, the downslope windstorm erupted on the leeward slope.