Summer mass mortality is one of the major bottlenecks limiting the green and high-quality development of the Pacific oyster Crassostrea gigas aquaculture in China. The frequent environmental stresses in summer, including high-temperature, rainfall, pathogen stimulation, picophytoplankton abundance and reproductive behavior, are important causes of mass mortality. Monitoring the changes of key environmental factors and health-related indicators in the aquaculture of C. gigas will help to forecast the risk of mass mortality. In this study, two surveys were conducted for C. gigas aquaculture in the North Yellow Sea during the summer heat and rainfall period to monitor the variations of water parameters, picophytoplankton abundance, bacterial abundance, glycogen content and immune-related indicators. It was found that, for the first and second survey in July, the surface water temperature was 25.3 and 24.9 °C, respectively. The salinity for the two surveys was 26.41 and 27.87, respectively. The abundance of diatoms for the two surveys was 3.6×10³ and 3×10³ cells/L, respectively, which was lower than the average level of previous reports, and the diversity of dinoflagellates increased. For the two surveys, the abundance of total bacteria in the water was 2.10×10⁹ and 2.77×10¹⁰ copies/L, respectively, and the abundance of Vibrio was 3.37×10⁸ and 5.40×10⁸ copies/L, respectively. The glycogen content of hepatopancreas decreased to 40.96 and 31.58 mg/g respectively for the two surveys, and the glycogen content of adductor muscle decreased to 6.63 and 8.91 mg/g, respectively. MDA content in gill was 13.74 and 8.69 nmol/mg prot for the two surveys, respectively, and T-AOC in gill was 0.46 and 0.77 U/mg prot, respectively. The expression levels of CgIL17-5 and CgTNF-1 mRNA in haemocytes dramatically increased in the first survey compared to the oysters cultured indoor. The expression levels of CgCaspase3 mRNA in haemocytes increased compared to the oysters cultured indoor, but the differences were not significant. In conclusion, the results showed higher water temperature and lower salinity happened in shellfish farming areas in the summer of 2021. The abundance of diatoms decreased compared to the average level of previous reports, and the diversity of dinoflagellates, a potentially harmful algae, increased. The abundance of environmental bacteria changed during the environmental stresses, and the risk of outbreak of potential pathogenic bacteria Vibrio increased. Organism responses were observed in oysters including lower glycogen content and mild oxidative stress. These results help us to further understand the environmental patterns and response characteristics of the organism during summer and provided a theoretical basis and reference for the prevention and control of the mass oyster mortality in summer.