In this paper, the meteorological observation data, NCEP reanalysis data, Himawari-8 satellite data were used to analyze a rare thunderstorm in the northwest semi-arid region from 8 to 9 October 2017. Its causes and mechanisms were discussed. The results show that the rare thunderstorm process originated from the eastern part of the Qinghai-Tibet Plateau, developed in the background of a ground warm center in front of the trough, and had the characteristics of surface-based convection. The thunderstorms seen in Lanzhou, about 150 km away from the surface front, occurred with a deep and stable cold air cushion. There were obvious inversion layers in the middle and lower layer, and warm and humid air with conditional instability above the inversion layer, which had the structural characteristics of elevated thunderstorms. This elevated thunderstorm process was different from the thunderstorm in the eastern humid area. In terms of its development and evolution, this elevated thunderstorm was actually the continuation and development of the surface-based thunderstorm in the Qinghai-Tibet Plateau, moving to the cold air cushion in lower elevation. In terms of the structural characteristics, the heights corresponding to each feature level (cold air cushion, inversion layer, etc.) in this process were significantly higher than those in the low-altitude areas in the east, but there was no significant difference in the thickness of each corresponding layer. In terms of the formation mechanism of the cold air cushion, the land-atmosphere energy exchange in the semi-arid area of the northwest was dominated by the sensible heat. With the combined effect of the unique underlying surface and strong cold air, the Loess Plateau with complex topography can also form a relatively stable cold air cushion. From the perspective of instability mechanism, the surfaced-based thunderstorm in east of the Qinghai-Tibet Plateau and the elevated thunderstorm were both caused by conditional instability. The structure of “cold and dry in the upper level, warm and wet in the lower level” was observed in the east of the Qinghai-Tibet Plateau. The updraft was induced under the combined action of high-altitude jet and 500 hPa high-level trough. In Lanzhou, the strong warm-wet advection in the middle troposphere enhanced the conditional instability above the inversion layer. The convergence effect of 700 hPa shear line cooperating with the synoptic-scale forcing of 500 hPa trough triggered the ascending motion and released the unstable energy, providing favorable conditions for the maintenance and development of vertical convection that moved eastward from the plateau to the cold air cushion.