[Objective] Microplastic pollution has become a global environmental issue that has caused widespread concern. Coastal zone is a region of land-ocean interaction under the dual impacts of global climate change and human activities. It is also an important sink for microplastics in the environment. However, by far little attention has been paid to long-term weathering characteristics of microplastics in coastal environments. In this study, microplastics of low density polyethylene(LDPE)film, which were commonly found in the coastal environments of the Yellow Sea and the Bohai Sea, were selected as the object for test in exploring long-term weathering characteristics of the microplastics in different coastal environments, including supratidal, intertidal and subtidal zones at the Yellow River Estuary. The aim of this study is to lay down a scientific basis for clarifying fates and effects of microplastics in the coastal environments.[Method] Microplastics of LDPE film were left in the tidal zones for exposure to the coastal environment for 12 and 18 months. At the end of the each exposure period, they were retrieved for analysis of surface morphology, chemical functional groups(carbonyl index)and density with scanning electron microscopy, Fourier transform infrared spectrometry, and pycnometry. Weathering degree of the microplastics was characterized.[Result] Results show that after 18 months of exposure in the field, colonies of microorganisms were observed on the surface of the microplastics in all the zones and so were significant weathering characteristics. The microplastics in the supratidal zone exhibited the highest degree of cracking. Carbonyl groups were also observed on the surface of the microplastics regardless of where they were in, and in terms of carbonyl index of the microplastics, the three zones displayed an order of supratidal zone (0.28-0.81)> intertidal zone (0.18-0.22)> subtidal zone (0.16-0.20). The microplastics did not show much spatio-temporal variation in density. However, having been subjected to ultrasonic cleaning for removal of surface attachments, the microplastics exposed aboveground at the supratidal zone for 18 months were by (0.85±0.02 g·cm^-3) significantly lower than the untreated ones (0.93±0.03 g·cm^-3) in density.[Conclusion] The dynamic changes in morphological characteristics, chemical functional groups (carbonyl index) and density of the microplastics of LDPE film indicate that weathering degree of the microplastics varying with the tidal zone shows a declining order of supratidal zone > intertidal zone > subtidal zone, and the microplastics in the supratidal zone varied sharply with time in weathering characteristics. The microplastics in the coastal environment are subjected to physical, chemical and biological weathering, such as light, friction, chemical oxidation, and biodegradation. Among them, light-triggered photochemical oxidation may contribute the most to the weathering of microplastics. In the future, more attention should be paid to the research on potential environmental effects and fate of microplastics as affected by the complex environmental factors in the coastal environment.