In order to understand the spatial and temporal variation of abundance of picoplankton, four seasonal cruises were carried out in Sanggou Bay, a typical large-scale mariculture bay in north China, in April, July, November, 2017 and January, 2018. We used flow cytometry to study the abundance of the main picoplankton (Synechococcus, picoeukaryotes and heterotrophic bacteria) and further analyzed the relationship between the abundance of those species and environmental parameters (such as temperature, nutrients, dissolved oxygen, etc.) Results showed that the average values of Synechococcus, picoeukaryotes and heterotrophic bacteria in the surface layer of Sanggou Bay was (2.93±2.29) ×10³cells/mL, (13.84±12.81) ×10³cells/mL and (1.03±0.28) ×10⁶cells/mL, respectively. There were significant differences in both abundance of picoplankton and spatial distribution between seasons. In spring, Synechococcuswas mainly distributed in the northwest, southwest and outside of Sanggou Bay; it was mainly distributed in outside of Sanggou Bay in summer and winter; in autumn, it mainly concentrated in the Northwest coast. Picoeukaryotes were concentrated in the western coastal area in four seasons, showing a decreasing trend from the inside to the outside of Sanggou Bay. There are two high-value areas of heterotrophic bacteria in outside and western coastal of Sanggou Bay in summer. In spring, autumn and winter, heterotrophic bacteria and picoeukaryotes had the same distribution trend, and it were concentrated in the western coastal area of Sanggou Bay. The correlation analysis between abundance of picoplankton and environmental factors showed that the abundance of three kinds of picoplankton had a significant positive correlation with chlorophyll-a, temperature and particulate organic matter, and was negatively correlated with dissolved oxygen. There was no significant correlation between Synechococcus and PO₄³⁻, NO₃⁻ and NO₂⁻. However, there was a significant positive correlation between picoeukaryotes and NO₃⁻ and NH₄⁺. The results provide data support for further understanding the structure and function of aquaculture ecosystem.