[Objective] This study was conducted to determine a rill erosion technology having high efficiency, high accuracy and strong adaptability, and overcome deficiencies associated with traditional measurement methods (low efficiency, high cost, poor applicability, etc.) in order to provide new ideas and technical means for the evolution of slope rill erosion and quantitative research. [Methods] UAV technology was used to acquire high-resolution images and models of rill development on slopes under six consecutive simulated rainfall events by use of nap-of-the-object photogrammetry. The feasibility of slope rill erosion and morphological evolution was quantitatively evaluated by positioning accuracy, model accuracy, and erosion simulation. [Results] ① The root mean square error (RMSE) of geographic registration of the 3D real scene model was 1.5 cm. The image control point plane RMSE was 0.42 cm. The elevation RMSE was 0.88 cm. The model details and texture were clear, with millimeter-level resolution. ② The multi-phase model clearly described five stages of rill development:raindrop splash erosion, sheet erosion, small waterdrop erosion, intermittent rill erosion, and continuous rill erosion. The average width, depth, and density of rills on the slope surface developed from the initial values of 1.25 cm, 0.82 cm, and 0.05 to the final values of 3.27 cm, 4.75 cm and 0.23, respectively. The maximum trench length was 236 cm and the maximum trench depth was 14.23 cm. ③ As rainfall duration increased, the simulated values of soil erosion in rills were close to the real values and tended to become stable, with an average error of less than 10%. [Conclusion] UAV nap-of-the-object photogrammetry can better reflect rill development and evolution, and has significant advantages over traditional measurement methods in efficiency and convenience.