Gully erosion, a dominant limiting factor in promoting ecosystem services and functions, is caused by a wide range of geo-environmental factors. However, limited information is available on the gully erosion mechanisms at a regional scale. Herein, susceptibility of a particular type of large-scale gullies across southern China were analyzed based on the 2005 National Survey on Soil and Water Loss. Specifically, frequency ratio model was used to determine the thresholds of 18 geo-environmental factors, including topography (elevation, slope gradient, slope aspect, slope length, hypsometric integral), distance from primary and tertiary rivers, climate (average annual temperature, precipitation, and rainfall erosivity), lithology, soil texture, soil moisture, vegetation index (NDVI, FVC, and EVI), as well as human activity (LULC and population density), followed by evaluating their relative importance by Boruta algorithm, and modelling the gully susceptibility modelling by random forest. The gully susceptibility was showed to be determined by comprehensive effect of all the 18 factors, with the top three important factors including average annual precipitation (9.97%), rainfall erosivity (9.60%), and distance from primary rivers (9.43%), whose thresholds were 1701 ∼ 1872 mm, 5967 ∼ 15676 MJ·mm·(ha·h·a)^-1, and 20000 ∼ 50000 m, respectively. Moreover, the random forest model with its AUC value ranging from 0.96 to 0.99 performed excellently in the spatial modelling of gully susceptibility. Collectively, the very low, low, moderate, and high susceptibility to these gullies occupied 65.30%, 21.36, 10.61% and 2.73% of southern China, in which the moderate and high susceptibility level were mainly clustered in and around Guangdong Province.