Numerical simulation is a normal method for slope stability analysis in open-pit mines. Slope model construction is the basis of numerical simulation, and the accuracy of topographic measurement data has an important influence on the accuracy of slope stability analysis. In recent years, low-altitude and low-speed small unmanned aerial vehicles (UAVs) have been widely used in geological surveys, which provide a feasible method for rapid terrain measurement under complex conditions. Considering the situation of the rapid change of slope shape of the open-pit mine, the complex terrain, and the difficulty of manual measurement, a single-lens small unmanned aerial vehicle (UAV) was used to perform low-altitude photogrammetry on an open-pit mine slope. Through UAV flight tests, the influence of the rotation angle of the UVA mounted lens on the slope measurement accuracy was compared and studied. Finally, a high-precision slope digital elevation model (DEM) was obtained, which was converted into a three-dimensional FLAC3D model using several softwares such as Surfer and Rhino. A rapid establishment was realized from the UAV measurement to the 3D numerical model. The following research conclusions can be obtained for reference: when the target slope is less than 45°, the rotation angle of UAV mounted lens has a large influence on the slope measurement accuracy. When the lens rotation angle is between 70° and 90°, relatively higher measurement accuracy can be obtained. Compared with manual measurement, the slope shape obtained by the UAV photogrammetry is more in line with the actual slope shape. After a series transformation of the aerial photos of UAV using the PhotoScan, Surfer, Rhino, etc., a 3D numerical model can be quickly built, and better numerical simulation results can be obtain to meet the needs of slope stability analysis.