Abstract: With the rapid development of the low-altitude economy and the increasing adoption of unmanned aerial vehicles (UAVs), urban low-altitude airspace is emerging as a critical scenario for future wireless communications. Millimeter-wave (mmWave) communication, enabled by its abundant spectral resources and capability for ultra-high data rates, is widely regarded as a key technology for achieving high-capacity and low-latency connectivity in low-altitude communication systems. To this end, 26 GHz millimeter-wave wireless channel measurements were conducted in a representative urban low-altitude scenario. Based on the measured data, the electromagnetic parameters of environmental materials were inversely estimated, leading to the calibration of material properties in the ray tracing (RT) simulation model. Based on the calibrated ray-tracing model, UAV flight trajectories at different altitudes were designed to investigate both large-scale (line-of-sight (LoS) probability and path loss(PL)) and small-scale (multipath cluster number, delay spread, and angular spread) channel characteristics. The results demonstrate that, with increasing flight altitude, the LoS probability gradually increases and the average path loss decreases, while the number of multipath clusters, delay spread, and azimuth angular spread exhibit a decreasing trend; in contrast, the elevation angular spread shows a moderate increase. Building upon these observations, key channel parameters, including LoS probability, path loss, multipath cluster number, and delay spread, are statistically modeled as functions of UAV altitude, leading to the development of an urban low-altitude millimeter-wave channel model applicable across different flight altitudes. The findings of this study provide both theoretical insights and quantitative support for link design, trajectory planning, and performance evaluation of urban low-altitude communication systems.