Abstract: Aerial-aquatic rotorcraft, capable of navigating in both underwater and aerial environments, has gradually become a focus of research in recent years. The physical properties of water and air differ significantly, thus the rotor experiences a complex two-phase flow environment during the cross-medium process. Using the lattice Boltzmann method (LBM), this study focuses on the aerodynamic characteristics and flow mechanisms of a rotor as it gradually approaches the water surface from the air. Combined with corresponding CFD results, it is shown that at a height of approximately 0.8 times the rotor radius above the water surface, the rotor enters a flow state similar to ground effect. As the rotor continues to approach the water surface, its downwash significantly interacts with the water, inducing the formation of a distinct liquid crown structure accompanied by strong vortex ring effects. This leads to a decrease in air pressure beneath the rotor, causing a continuous decline in rotor lift during this period. The lift characteristics exhibit a turning point, where the lift decreases instead of increasing, indicating significant gas-liquid two-phase flow interference. After the liquid crown structure breaks up, the rotor lift stabilizes. At H/R = 0.2, the rotor lift increases by approximately 26% compared to its performance in the air. At H/R > 0.4, the growth rate in rotor lift gradually slows, decreasing at a rate of 2% to 5%, until it eventually converges to the lift level observed in the aerial condition. This paper preliminarily reveals the initial patterns of the thrust characteristics of rotors under near-water surface conditions and their influencing mechanisms, providing a reference basis for subsequent theoretical research and practical applications.