Microplastic transport in marine systems is a topical research hotspot in recent years. Despite extensive studies, the cross-domain transport mechanisms linking estuarine and open-sea microplastics remain poorly understood. This study investigates microplastic migration patterns across the estuary-open ocean continuum in the northern South China Sea based on in-situ observations. In the Pearl River Delta estuary, microplastic abundance was higher in wet seasons, whereas no statistically significant seasonal discrepancy was detected due to insufficient sampling coverage and high sample heterogeneity. A weak correlation between microplastic abundance and precipitation further suggests that terrestrial runoff exerts a marginal effect on estuarine microplastic distribution. For the offshore basin, the horizontal microplastic abundance within anticyclonic eddies was negatively correlated with eddy kinetic energy, and the vertical migration velocity of microplastics increased exponentially with Ekman current velocity. These results indicate that anticyclonic eddy dynamics serve as a key driver modulating the horizontal advection and vertical redistribution of offshore microplastics. Future work will adopt high-resolution numerical simulations to quantify the vertical transport kinetics of marine microplastics.