Slope-gully model system of the Loess Plateau was established based on the controlled scouring experiments. Study of the effects of slope length on the erosion power factors of slope-gully system could provide scientific guidance for the mechanism of water erosion process in slope-gully system. This study set up five water flow rates (6, 8, 10, 12, 14 L/min) and three slope lengths (4, 6, 8 m). The results showed that the Reynolds number increased with the increasing water flow rates acrossing each section of the slope-gully system, and the Reynolds number in the slope was higher than in the gully. Froude number also increased in the slope-gully system and the increased rate of Froude number in the gully was obviously higher than that in the slope. The Froude number had the highest value when the water flow rate was 10 L/min. The runoff velocity in the gully was obviously higher than in the slope under the same water flow rate, and the runoff velocity in the slope was much stable than in the gully. The resistance coefficients sharply reduced to 0.03～0.05 from section 2 under different slope length and water flow rate of 10 L/min. The fluctuation of resistance coefficients was less with the decreased slope length under the same water flow rate (except for 10 L/min). Runoff erosion power was significantly correlated with Reynolds number, Froude number, runoff velocity. The relationship between runoff erosion power and Reynolds number was a logarithmic function, while the power functions occurred between runoff erosion power and Froude number, flow velocity and resistance coefficients. These results showed that the slope length obviously affected the erosion hydraulic parameters in the slope gully system, and these findings could provide scientific bases for the research of erosion energy transfer in the slope-gully system.