To explore the impact of grassy slope fire prevention project on runoff and sediment yield on grassy slope in arid valleys area, a series of sourcing experiments were conducted in three steep slopes of 27.5° in Liangshan Prefecture of Sichuan Province. The flow rates of sourcing experiments were 6, 9 and 12 L/min, respectively. The three slopes were treated as follows: (1) Remove vegetation and level the slope to represent the grassy slope fire passage; (2) Remove the aboveground part of the vegetation to represent the grassy slope fire isolation zone; (3) No treatment as a control. The effects of different grassy slope fire prevention project on water erosion under different scouring flow rates were studied by collecting the runoff and sediment at the slope outlet, respectively. The results showed that: (1) Due to the loose soil on the slope and the increase of water infiltration during the construction of the grassy slope fire prevention project, the initial runoff time of the grassy slope fire passage (20.33 min) was larger than that of the control slope (7.62 min) and the grassy slope fire isolation zone (5.22 min). (2) The order of the cumulative runoff and cumulative sediment yield under the three flow rates were as follow: grassy slope fire passage > grassy slope fire isolation zone > control slope. Moreover, the difference between the three slopes increased significantly with the increase of the flow rates. The construction of grassy slope fire prevention project had aggravated the water erosion on the steep slope. (3) There were linear relationships between the runoff and sediment yield on the control slope, while the relationships between the runoff and sediment yield were exponential, logarithmic, or linear for the grassy slope fire passage. The relationships between the runoff and sediment yield for the grassy slope fire isolation zone were logarithmic or linear. The construction of the grass slope fire prevention project has changed the original linear relationships between runoff and sediment yield. Therefore, more attention should be paid on soil erosion after the construction of grassy slope fire prevention project on steep slopes. This research can provide scientific reference for the impact of grassy slope fire prevention project on soil erosion in arid valleys and the implementation of preventive measures on steep slopes.