Abstract: Objectives: Gully development has become a critical issue in the mollisol region in northeast China, severely damaging black soil resources and threatening national food security. Ephemeral and permanent gullies are the main erosion gullies in the mollisol region in northeast China. Currently, the control and management of gully erosion in the mollisol region in northeast China mainly focus on treating permanent gullies, with insufficient attention on preventing ephemeral gullies development into permanent gullies. The fundamental reason lies in the inadequate scientific understanding of this dynamic process. This research aims to offer a quantitative analysis of ephemeral gullies developing into permanent gullies in recent years in a typical watershed in the mollisol region of northeastern China. Methods: This study analyzed the phenomenon of ephemeral gullies developing into permanent gullies in the Hebei Watershed in Nenjiang County, Heilongjiang Province, covering an area of 28.02 km². The morphological parameters of ephemeral gullies and permanent gullies from 2011 to 2022 were obtained through visual interpretation based on high-resolution satellite images and unmanned aerial vehicle (UAV) images, and the accuracy was verified by filed measurement data using Global Navigation Satellite System-Real Time Kinematic (GNSS-RTK). The slope and draining area were obtained using the Digital Elevation Model (DEM) with a 1m resolution derived from the UAV image. Plotting slope and drainage area data on a double logarithmic coordinate system to construct a slope-area critical terrain threshold model. Results: From the analysis of the phenomenon of ephemeral gullies developing into permanent gullies, the development rate, distribution of vulnerable areas, and critical terrain parameters were acquired. The results showed that: (1) A large number of ephemeral gullies in the study area developed into permanent gullies from 2011 to 2022, accounting for 23.7 % of the total length of ephemeral gullies in 2011. By 2022, the number and total length of permanent gullies had significantly increased, reaching 5.5 times and 5 times greater than those in 2011, respectively. Approximately 50% of these permanent gullies developed from ephemeral gullies during the period, with a length change rate of 16.6 m·a^-1·km^-2; (2) The development of ephemeral gullies into permanent gullies mainly distributed intermittently in the lower middle parts of the left and right banks as well as in the downstream areas of gully palm within the sub-watersheds in the Hebei watershed, with a few distributed on both sides of the main channel, and longer ones in the downstream areas of gully palm; (3) An area-slope critical threshold model of watershed for the transformation of ephemeral gullies into permanent gullies was constructed, with the model formula as S≥ 0.421 A- 0.252 , and the accuracy of the model was 72.7 %. Conclusions: The high-resolution satellite image and high-precision modern surveying and mapping technology played an essential role in this research. Ephemeral gullies were rapidly developing into permanent gullies in the Hebei watershed in the mollisol region of northeastern China. The critical terrain threshold model can predict the locations that are vulnerable to occur. Further study on the accuracy and applicability of critical terrain threshold model construction can be improved through more factors analysis in regions of different erosion types.