Abstract: This study conducted a four-quarter survey of microplastic pollution in Qinzhou bay, a farming bay in southern China, and its upstream rivers, ranging from remote areas in the upper reaches of the river to economically developed coastal cities. The annual average abundance of microplastics in the Qin river and Maoling river was 3.94 particles per liter and 1.63 particles per liter respectively. In the estuarine area of Qinzhou bay and the outer bay, the annual average abundance of microplastics was 1.76 particles per liter and 1.50 particles per liter respectively. The annual average microplastic pollution intensity was ranked in the order of Qin River, Maowei Sea , Maoling river, Outer bay. Significant seasonal differences occurred, notably in Qin river (p < 0.01), with winter abundance five times higher than summer; outer bay showed least seasonal effect. In winter, microplastic abundance in the Qin river was higher than in other areas; in summer, microplastic abundance in the outer bay was higher; and in spring and autumn, microplastic abundance in the middle reaches of the Qin river was higher. The predominant color of microplastics in upstream rivers was blue, while those in estuaries and the outer bay were mainly transparent. The main types of microplastic polymers were rayon and PET. It is noteworthy that microplastic abundance did not significantly increase from upstream to downstream rivers but showed a positive correlation with the coastal population. Microplastics in the Qinzhou bay watershed mainly originate from the daily activities of coastal residents and fisheries activities within the watershed, while they may also be influenced by environmental factors such as surface runoff and rainfall.