Abstract: As emerging environmental contaminants, the flux and mechanisms of microplastic transport from terrestrial areas to water bodies represent a critical focus of current research. Within this context, an in-depth investigation into the distribution differences of microplastics between aquatic and terrestrial compartments in high-mountain headwater catchments is essential for a comprehensive understanding of their source-to-sink processes. This study was conducted in the Yunling Lasha Mountain catchment, a tributary of the Lancang River. By collecting both aquatic sediments and terrestrial soil samples, we aimed to reveal the distribution differences, compositional characteristics, and transport patterns of microplastics within this catchment. The results indicate that: (1) The overall level of microplastic pollution in the catchment was relatively low, with average abundances in aquatic sediments and terrestrial soils of (30.74±16.97)×10⁴ items/m³ and (19.81±18.41)×10⁴ items/m³, respectively; (2) No significant difference in microplastic abundance was found between aquatic and terrestrial compartments in the upstream area, whereas abundances in midstream and downstream aquatic sediments were significantly higher than in adjacent soils, confirming surface runoff as a key process driving terrestrial-to-aquatic translocation and enrichment; (3) The particle size of aquatic microplastics showed a fining trend longitudinally along the river, with increasing proportions of small and medium-sized particles and a decreasing proportion of large particles, indicating physical fragmentation caused by hydraulic abrasion; (4) Analysis of color and shape composition revealed that blue, black, and white fibers and fragments were the overwhelmingly dominant types, suggesting that the microplastics in this catchment primarily originate from daily life and agricultural activities. This study confirms that even in a high-altitude catchment with minimal direct human disturbance, natural hydraulic processes remain a key factor driving the terrestrial-aquatic translocation and downstream evolution of microplastics. These findings enhance the understanding of microplastic transport behavior in natural environments and provide a scientific basis for pollution control in headwater regions.