To reveal the influence mechanism of frequent small-fluctuation transient processes on the hydraulic stability of hydroelectric units, a 1D-3D coupling method was adopted to conduct a full-passage numerical simulation of the transient process of an 800 MW-class Francis turbine when its load increased from 55.6% to 61.2% of the rated power. This study focused on the guide vane regulation transient process, the vortex evolution laws in the runner and draft tube regions, and the pressure pulsation characteristics. The results show that as the guide vane opening increases, the discharge grows steadily, the axial force undergoes a sudden change due to the water hammer effect, and the radial force, affected by the guide vane regulation disturbance, exhibits typical low-amplitude pulsation characteristics, though its amplitude is significantly larger than that under steady operating conditions. As the load increases, the scale of the inter-blade vortex gradually decreases. When the load reaches 61.2% of the rated power, the inter-blade vortex basically dissipates, and the draft tube vortex rope evolves from a single vortex rope structure into a stable double vortex rope structure at the center of the draft tube. During the load-increasing process, the unit is affected by the coupling of water hammer, vortex structure evolution, and rotor-stator interaction frequency, generating high-amplitude frequency components such as 0.11 f n, 0.21 f n, 0.85 f n, and 15 f n.