Abstract: This paper investigates the propagation mechanism and time-varying fading characteristics of air-to-air channels for maritime unmanned aerial vehicle (UAV)-to ship broadband communications under evaporation duct conditions. The refractive index profile of the near-sea low-altitude evaporation duct is approximated as a linear function. Based on Snell’s law and the variational principle, a closed-form expression for the curved propagation path is derived. By incorporating sea-surface and duct reflections, five propagation modes are identified, and a multi-mode multi-hop propagation model is established. An adaptive step-based algorithm combining coarse and fine searches is proposed to efficiently determine the initial launch angles of multipath components, and the corresponding path parameters are calculated to construct a broadband channel model with multimode multipath superposition. Simulation results show that, under evaporation duct conditions, radio waves propagate along curved trajectories bending toward the sea surface rather than straight lines, leading to noticeable differences in delay, angle of arrival, and angle of departure compared with conventional straight-line models. The multipath propagation pattern evolves with distance. Under conditions where the transmitter–receiver separation does not exceed 20 km and the carrier frequency is 28 GHz, the delay difference between curved and straight propagation paths becomes non-negligible when the bandwidth exceeds 0.5 GHz. Meanwhile, as the transmitter–receiver distance further increases, the line-of-sight path gradually disappears, and the multipath progressively evolves into a propagation mode dominated by sea-surface reflections and evaporation-duct-guided propagation. The proposed model effectively characterizes multipath propagation and time-varying fading in evaporation duct environments and provides theoretical support for link design and anti-fading techniques in maritime UAV communication systems.