Abstract: Gully erosion has been the most severe form of slope soil erosion in arid and semiarid areas. It is very necessary to accurately monitor the morphological characteristics for the prevention of gully erosion. Typical indicators can be used to describe the morphological characteristics of the gully development, including the gully length, width, depth, breadth depth ratio, and the vertical slope of the gully. Among them, the gully length has not yet been known in the stage of gully erosion so far. Alternatively, a low-cost and high-precision topographic monitoring technique, oblique photography has a wide range of applications in the detection of gully erosion in recent years. In this study, 82 sections of 10 gullies were selected from four sites in the hilly and gully region of the Loess Plateau area in China. The indexes of gully length, width and depth were extracted from the field measurements and Unmanned Aerial Vehicle (UAV) oblique photography with a real-time kinematic and global navigation satellite system (GNSS RTK). A volume estimation model was then established to determine the gully length. The results show that there were various indicators for the different accuracy and applicability of gully morphological parameters using oblique photogrammetry. Specifically, 1) compared with the field measurements. The deviation extent (DE) of gully length and gully width was less than 4%, indicating a higher accuracy than before. The mean values of DE of gully depth were greater than 25%, indicating a larger error. Therefore, the UAV oblique photography with the GNSS RTK was highly feasible to measure the index of gully length and gully width, but there are certain limitations in the application of gully depth measurement with high accuracy requirements. 2) There was a significant power function relationship between the gully volume and gully length. The volume-length model was also established to predict the development stage of the gully erosion, in terms of various parameters during simulation. The parameter values of the model show that the selected gully in the study area presented a high average development degree, particularly with the large cross-sectional area, deep erosion degree, and small development potential. The growth of gully volume was mainly controlled by the length of the gully, indicating a weak relationship with the gully width and depth. The model fitting demonstrated the best imitative effect when the gully length was selected as the horizontal projection length of the maximum catchment path, followed by the length of the gully centerline, and the linear distance from the gully head to the gully exit was the worst. Consequently, it was feasible for the line distance as the gully length during field measurements when the slope was less than 18.2°. And more importantly, the volume-lengh model should fully consider the influence of the development stage in the gully erosion.