Abstract: To explore the impact of vegetation density on hydrodynamic characteristics of runoff on engineering steep slopes, based on field simulated rainfall experiments, bare slopes (Bare slope, BS) were used as controls to analyze the hydrodynamic characteristics of two densities of Paspalum notatum Flugge (PF10:10 cm×10 cm, PF20:20 cm×20 cm) under three rainfall intensities and their differences from bare slopes, revealing the dynamic mechanism of vegetation-protected steep slope erosion. The results showed that: ① Both bare slopes and vegetated steep slopes exhibited laminar flow (Reynolds number < 100), but the flow pattern varied between slow and rapid flows depending on different rainfall intensities. PF10 and PF20 reduced the average Reynolds number by 54.49% and 5.95%, respectively, and reduced the average Froude number by 41.69% and 33.80%, respectively. ② When rainfall intensity was less than or equal to 1.5 mm/min, vegetation reduced flow velocity, but at 2.0 mm/min, PF20 increased flow velocity while PF10 still reduced it. The benefit of PF10 in reducing average flow velocity was 1.98 times that of PF20. However, both densities of vegetation significantly increased the runoff resistance coefficient, with PF10 increasing the resistance coefficient by 1.47 times that of PF20. ③ PF10 reduced the average runoff shear stress and runoff power by 23.06% and 54.89%, respectively, while PF20 reduced the average runoff power by 9.79% but increased the average runoff shear stress by 22.87%. From the aspect of runoff power, an optimal hydrodynamic parameter for characterizing steep slope erosion, the erodibility of PF20 and PF10 was 98.44% and 48.43% of that of the bare slope, respectively. The research results can provide scientific guidance for vegetation restoration on engineering steep slopes.