Abstract: Understanding the real degradation of biodegradable plastics is crucial for comprehensively evaluating their environmental safety. In this study, the degradation characteristics of poly(butylene adipate-co-terephthalate) (PBAT) in offshore environment through field and laboratory researches were investigated. After one year of exposure in the East China Sea, three types of PBAT-based products showed breakage and holes on the surface, and the infrared signals of the oxygen-containing functional groups and hydroxyl groups increased, but their thermal stability decreased. There was a poor degradation of plastics possibly due to low temperature of seawater and the attachment of lots of algae and minerals to the surface. However, a degradation experiment in laboratory found that strong ultraviolet irradiation accelerated the aging of PBAT mulching films. Both the weight loss rate of plastics and the number of released microplastics increased as the irradiation period, and reached 44.1%±3.68% and 67.1±18.8 items/mg after UV radiation for 84 h (equivalent to approximately 340 days in nature), respectively. Dissolved organic matters (DOM) were also released from the films during aging and main fluorescent organic compounds included tyrosine, protein-like substances, and aromatic proteins, but photodegradation occurred due to their photo lability. Therefore, the content of dissolved organic carbon released from PBAT initially increased and then decreased.