Gully erosion is one of the main modes of slope erosion on the Loess Plateau, which plays a connecting role in the slope gully erosion system. The Loess Plateau has wide and densely distributed gullies. The study selected a typical small watershed in the hilly and gully region of the Loess Plateau to measure the morphological characteristics and spatial-temporal distribution of gullies. A deep learning image semantic segmentation model was used to identify and extract the morphological features of gullies at the watershed scale from 2009 to 2021 based on remote sensing images (0.5 m resolution) and then analyze their temporal and spatial distribution characteristics. The results revealed that: (1) most gullies occurred in the hilly southern parts of the watershed, which has complex landforms and large slope gradients; (2) gully number increased from 1,159 in 2009 to 2,312 in 2021 (average 97 per year), with a frequency development rate of 2.87 km ^–2 y ^–1; (3) gully length generally ranged from 25–40 m, with an average growth rate is 1.66 m y ^–1 and density development rate of 0.12 km km ^–2 y ^–1; (4) gully width ranged from 0.5–1.5 m, with an average growth rate of 0.04 m y ^–1. (5) the total gully area increased from 0.0566 km² in 2009 to 0.1072 km² in 2021, with a development rate of 4,213.39 m² y ^–1 and dissection degree development rate of 0.0125% y ^–1. This study provides a theoretical and scientific basis for gully erosion control and eco-environmental protection at the watershed scale on the Loess Plateau.