Abstract: The sources of microplastics in coastal areas are complex. The fragmentation of large plastics through physical, chemical and biological processes is one of the main sources of microplastics in the beaches. This study investigated the compositions, abundances, shapes, colors and components of plastic debris and the microplastics of different sizes in 7 beaches in Sanggou bay of the Yellow Sea. We also studied the relationships between the fragmentation of plastic debris and microplastics by qualitative and quantitative analysis, which can be used to identify the sources of the microplastics. The results showed that polystyrene foam accounted for 85%, 97% and 82% of the samples in the size range of 5 mm ~ 2.5 cm, 1 ~ 5 mm and <1 mm respectively. In plastic debris >2.5 cm, the types of the plastic debris related to mariculture accounted for 76%. The total abundances of plastic debris and microplastics increased by 2 ~ 4 orders of magnitude with the decrease of sizes.The abundances of microplastics and plastic debris in 1 ~ 5 mm and 5 mm ~ 2.5 cm were significantly correlated. By comparing the morphology and components characteristics of different sizes of plastic debris and microplastics, the sources of microplastics were identified as fragmentations from polystyrene foam, polyethylene float, yellow sponge, fiber fishing ropes and polypropylene film ropes. The results indicate that mariculture is one of the important sources of plastic debris and microplastics in beaches of Sangou bay. In the future, it is necessary to further explore the identification methods of sources and find new evidences to identify the sources of microplastics.