Abstract: By investigating various environmental factors and phytoplankton in specific marine areas, this study serves as a reference to determine whether the lower water environment can support the growth of shellfish, based on traditional upper shellfish culture practices. In spring (end of March) and autumn (end of September) 2022, relevant tests were conducted on seawater in the upper (2 m) and lower (8 m) layers of shellfish culture areas in Longkou City (K1, K2) and Laizhou City (L1, L2) in the eastern part of Laizhou Bay, Shandong Province. The results indicated that water temperature, pH, salinity, dissolved oxygen (DO), chemical oxygen demand (COD), and labile phosphorus (DIP) in both upper and lower layers of the four culture areas met the national second-class water quality standards. Nitrite, nitrate, and ammonia nitrogen (NH₄-N) concentrations were within safe limits. N/P ratio analysis revealed phosphorus limitation in both upper and lower layers, except for suitable nitrogen-to-phosphorus ratios under L1 in spring and K1 in autumn. Phytoplankton density was higher in the upper layer than the lower layer spatially, higher in spring than autumn, with lower shellfish food availability in Longkou City during autumn. Longkou City exhibited rich phytoplankton in both upper and lower layers during spring, as well as in both seasons of Laizhou City. Considering cultural literature and the indicators in this paper, it is suggested that the water quality and phytoplankton density in the upper and lower layers of Longkou City and Laizhou City during spring and autumn may provide opportunities for expanding the water layer in shellfish culture. However, the higher total phosphorus content in the lower layer of Longkou City raises concerns about exacerbating eutrophication, coupled with relatively low phytoplankton density in autumn, which may not be conducive to shellfish growth. This study, through water quality investigation, aims to offer data support for dynamically adjusting and optimizing the rational layout of shellfish culture.