Abstract: Based on aircraft observation data collected over coastal East China on September 23, 2023 combined with reanalysis data and a backward trajectory model, this study comprehensively analyzed the vertical distributions of aerosols, cloud condensation nuclei (CCN), and cloud microphysical properties to investigate aerosol–cloud–precipitation interaction during a weak precipitation event. The main findings are as follows: (1) An elevated aerosol layer was observed at 1.5–3 km with a peak number concentration (N_a) of 6500 cm⁻³ and a uniform effective diameter (D_a) of ～0.11 μm. This layer coincided with a temperature inversion, and the cloud microphysical parameters indicated the presence of large particles, likely remnants of evaporated cloud droplets at this level. Backward trajectory analysis suggested that this high-concentration aerosol layer may have originated from evaporated cloud droplets in upstream cloud regions. (2) The CCN activation spectra exhibited significant vertical variation, with aerosol activation characteristics transitioning from a marine–continental mixed type in the lower atmosphere to a cleaner continental type at higher altitudes. In addition, the aerosol activation ratio decreased exponentially with increasing aerosol concentration under the same supersaturation conditions. (3) High liquid water content (LWC; peak value of 0.79 g m⁻³) coexisted with high cloud droplet concentration (N_c; exceeding 3000 cm⁻³) within the cloud. Notably, in regions where LWC exceeded 1 g m⁻³, small droplets (D_a<15 μm) accounted for more than 90% of the droplet spectrum, with no significant growth observed at the large-droplet end. This indicates that cloud droplet growth was dominated by condensation, while collision–coalescence processes were suppressed, leading to the low precipitation efficiency. Overall, this aircraft-based observation case study under relatively clean conditions over coastal East China suggests that cloud evaporation may be an important source of the high-altitude fine-mode aerosol layer. Meanwhile, the condensation-dominated microphysical structure within the cloud may have constrained the development of precipitation efficiency. These findings provide valuable observational insights into aerosol–cloud–precipitation interactions under similar weak precipitation conditions in coastal East China.