Purpose Gully erosion is one of the major contributors to severe land degradation in the Yuanmou Dry-hot Valley Region, Southwest China. Grass has been proved to have great advantages in gully erosion control. This study aimed to detect the influence of grass basal diameter on change processes of runoff velocity, sediment transport rate, and runoff energy consumption. Materials and methods A series of scouring tests were conducted based on a field experimental platform under the same flow discharge, and the impacts of different grass basal diameter (d) (0, 17, 43, 70, and 98 mm) on variations of runoff velocity (v), sediment transport capacity (S-t), and runoff energy consumption (E-c) were explored in this study. Results and discussion The results showed that values of runoff velocity and sediment transport capacity decreased notably with the increase of grass basal diameter, and two declining power functions can be found (v = 0.689*d<^>(- 0.106), p < 0.01; S-t = 0.741-2.228E-11*d<^>4.701, p < 0.01). As for the spatiotemporal variation, the influence of grass basal diameter on runoff velocity had obvious stage features, and the influence on sediment transport capacity mainly concentrated on spatial variation in the downstream of gully bed (16(th)-20(th) m away from gully head). Furthermore, the increasing of grass basal diameter could effectively enhance runoff energy consumption, and a logistic growth function had been found. Finally, the grass basal diameter of 70 mm proved to be the critical grass basal diameter for effectively reducing runoff erosion force and increasing runoff energy consumption in gully beds in this study. Conclusion These results indicate that the grass growth can effectively reduce runoff erosion force and increase runoff energy consumption, and in general, this function will be enhanced with the improvement of grass basal diameter in gully beds.