Abstract: Extreme rainfall events have severely exacerbated the risk of gully erosion on the sloping landscapes of the Loess Plateau. Their ever-increasing frequency and intensity require clarification of the development of newly formed gullies and their underlying controlling factors. Soil and water conservation can be optimized to mitigate severe soil degradation in semi-arid regions. In this study, a systematic investigation was conducted to evaluate the morphological characteristics and influencing factors of the newly formed gullies triggered by an extreme rainstorm event in July 2025 in Xin'anbian Town, Dingbian County, northern Shaanxi, China. Three types of slope land use were selected: sloping cropland, terraced cropland, and terraced grassland. Spearman correlation analysis was used to identify the statistical relationships between gully morphological parameters and environmental factors within the contributing catchments. Path analysis models were subsequently constructed to quantify the direct and indirect effects of the key environmental factors. The results showed that the different types of slope land use significantly influenced the development of newly formed gullies. Sloping cropland exhibited the greatest erosion magnitude, with mean gully volume, length, surface area, and depth of 54.47 m³, 53.34 m, 58.12 m², and 1.18 m, respectively. In contrast, terraced cropland and terraced grassland markedly reduced the overall erosion magnitude: their mean gully volume, length, and surface area reached only 9%–12% of those on sloping cropland, while mean gully depth decreased to 54%. There was a relatively small difference in the catchment areas of terraced cropland and terraced grassland. Gully development was further reduced under terraced grassland, indicating an additional regulation of vegetation restoration. In cross-sectional morphology, mean gully widths were 1.90, 1.20 and 1.25 m, respectively, on sloping cropland, terraced cropland, and terraced grassland, while mean cross-sectional areas were 3.91, 0.78 and 0.83 m², respectively. Gullies on terraced slopes generally exhibited wider and shallower cross sections, with the width-to-depth ratios of 15%-23% greater than those on sloping cropland. Correlation and path analyses further revealed that gully development was regulated by catchment area, soil penetration resistance, and physical crust thickness. Catchment area was the dominant controlling factor on gully enlargement, indicating a significant direct positive effect on gully volume, accounting for 93.0% of the total direct effect. There was a significant influence of catchment areas on soil penetration resistance, indirectly promoting the development of gully cross sections. Physical crust thickness shared a significant direct negative effect on gully cross-sectional areas, whereas soil penetration resistance primarily regulated gully development to modulate soil erosion resistance. There was a great variation in the runoff concentration of the newly formed gullies under extreme rainfall. Catchment area was primarily responsible for the overall enlargement of gullies. Soil physical properties and surface protection further regulated channel morphology. A synergistic control mechanism was involved with runoff concentration, soil properties, and surface protection in the newly formed gullies under extreme rainfall. The findings can also provide a scientific basis to improve gully erosion control for soil and water conservation on the Loess Plateau.