Abstract: On 29 June 2023, a local rainstorm process occurred in the Wumeng Mountains in northwest Guizhou. The rainfall intensity was high, the time was concentrated. The maximum hourly rainfall intensity reached 118.6 mm, causing severe casualties and significant property damage. Using conventional meteorological observations, radar data, and ERA5 reanalysis data, the heavy rainfall event are diagnosed and analyzed with the methods of bottom wind field decomposition and frontogenesis function. The results showed that: (1) Due to the combined effects of the upper trough and low-level shear, the heavy rainfall process was caused by the frontogenesis of convergent line. Significant convective instability and abundant water vapor occurred in the northwestern Guizhou before the heavy rainfall. (2) The heavy rain area could be divided into northern and southern segments. The northern segment was quasi east-west direction, the strong center distribution was even, the movement of the strong rain area and the convective echo were consistent with the trend of the mountains. The southern segment showed ring-shaped, which was in the windward slope of Wumeng Mountain, exhibiting intense rainfall with backward-propagating convective echoes and a “train effect.” (3) The terrain caused the development and strengthening of the surface convergence line in the rainstorm area. The low-level southeasterly flow appeared both upslope flow and around-flow along the gradually rising terrain in the northwest of Guizhou. The upslope flow strengthened the ascending motion in the low-level. A dense area of θse was formed along the windward slope, resulting in frontogenesis. The above analysis showed that the effect of topography could cause the strengthening of rainfall, especially in the southern section of the windward slope area, where extreme, disaster-causing rainfall occurred.