The study of mechanisms of gully headcut erosion is crucial for predicting and preventing soil erosion and effectively reducing gully erosion. However, quantitative analysis from multiple perspectives and dimensions of gully morphological characteristics and their evolution patterns is still unclear. This study conducted a series of indoor flushing experiments using gully headcuts of different heights (flow rates of 2, 4, and 6 L min-1; gully headcut heights of 5, 10, and 15 cm). Based on DEM and monitoring data, this study utilizes three erosion modes: gully headcut retreat erosion, gully wall widening erosion, and gully bed downcutting erosion, as representative factors for the longitudinal, lateral, and vertical dimensions, respectively. Through these factors, the characteristic patterns of three distinct dominant developmental modes during gully headcut erosion are revealed. The results indicate that: (1) Discharge rate significantly impacts changes in gully length and width, increasing average headcut retreat erosion rates by at least 19.5% and average headcut erosion width by at least 10.4%. Gully headcut height notably affects gully bottom incision depth, reducing average gully bottom elevation by at least 21.0%. (2) The farther away from the top of the slope, the larger the width of the gully caused by the collapse of the gully wall, and the average widths of the collapse on the slope, during the slope and under the slope were 0.9, 1.9, and 3.8 cm, respectively, indicating that the collapse of the slope is more likely to occur under the slope. (3) The slope of the gully bottom increases with the height of the gully headcut. As the discharge increases, the morphology of the gully bottom slope transitions from "convex upwards" to "concave downwards". (4) In early stages of erosion development, headcut retreat erosion predominates, contributing 53.2%-82.7% of sediment production. In middle and later stages, gully wall expansion and gully bottom erosion and deposition dominate. These research findings provide insights into mechanisms governing gully erosion on loess slopes.