| In order to better understand the spatial and temporal variation of abundance of Synechococcus, Picoeukaryotes and Heterotrophic Bacteria, four seasonal cruises were carried out in Sanggou bay, a typical large-scale mariculture bay in north China, during the period from April in 2017 to January in 2018. The relationship among the abundance of those species and other environmental parameters (such as temperature, nutrients, dissolved oxygen, etc.) were further analyzed. Results showed that the average values of Synechococcus, Picoeukaryotes and Heterotrophic Bacteria in the surface layer of Sanggou Bay was (2.93±2.29) ×103cell/ml, (13.84±12.81) ×103cell/ml and (1.03±0.28) ×106cell/ml, respectively. There were significant differences in both abundance of picoplankton and spatial distribution between seasons (P<0.01). In spring, Synechococcus mainly distributes in the northwest, southwest and outside of Sanggou Bay; it mainly distributes in outside of Sanggou Bay in summer and winter; in autumn, it mainly concentrates 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. In summer, Heterotrophic bacteria have a high abundance in outside of Sanggou Bay and it and Picoeukaryotes have the same distribution trend in other seasons. The correlation analysis between abundance of picoplankton and environmental factors showed that the abundance of three kinds of picoplankton was a significant positive correlation with chlorophyll-a, temperature and particulate organic matter, and 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.