Abstract: Oyster Crassostrea ariakensis is one of the typical bivalve molluscs and is an important economic species in China. It mainly grows in the estuary in China and occupies an important position in the bivalve industry in Guangdong. In this study, C. ariakensis was selected as the experimental organism to explore the impact of the polystyrene microplastics on the immunity of C. ariakensis. In order to study the effects of microplastics immunity and antibacterial activity of oyster, the oysters (average shell length: 6.6 cm) were injected with different concentrations of polystyrene microplastics, respectively (0.05 mg/L, 0.5 mg/L, 5 mg/L, with sterilized saline water as diluent) by intra-shell injection (without damage to the soft tissue of oyster). The gills were sampled after being treated with microplastics for 24 h and 72 h, respectively, activities of six immune related indexes, including alkaline phosphatase (AKP), superoxide dismutase (SOD), malondialdehyde (MDA), catalase (CAT), nitric oxide synthase (NOS), acetylcholinesterase (A-chE) and the mortality rates were recorded. The results showed that the activities of SOD and A-chE in the gill of oysters were significantly affected by microplastic concentration after 24 h (P<0.05). The activities of AKP increased first and then decreased, while SOD and MDA showed an upward trend, A-chE and NOS showed a downward trend. After 72 h of microplastic treatment, the enzyme activity was significantly lower than those of 24 h (P<0.05). With the increase of microplastic concentration, the trend of changes in enzyme activity is roughly consistent with 24 h trend. When the concentration of microplastics was above 0.5 mg/L, the activity of NOS was lower than that of the control group, and the cumulative mortality of oysters was significantly higher than that of the control group (P<0.05). The above results showed that with the increase of microplastic concentration, the oxidative stress of oysters was more intense. With the extension of exposure time, the antibacterial ability and immune ability of oysters decreased. It is expected to provide reference data for the analysis and evaluation of the potential risks of microplastics to the filter-feeding bivalves.