Three cruises were conducted in May, July, and November 2016 in artificial reef areas constructed in 2010, 2012, and 2014, respectively, and the non-artificial reef area (control area) offshore of Tianjin with three sampling stations for each, totaling 12 sites. The results identified 58 taxa of phytoplankton belonging to 2 phyla and 28 genera. Among these species, Bacillariophyta includes 19 genera and 44 species, and Pyrrophyta includes 9 genera and 14 species. Diatom species are dominant, accounting for 75.9% of all species, followed by dinoflagellates, accounting for 24.1%. Dominant phytoplankton species are Coscinodiscus and Chaetoceros of Bacillariophyta, mainly Coscinodiscus wailesii, C. asteromphalus, C. granii, C. oculus-iridis, C. spp., Chaetoceros castracanei, Ch. lorenzianus, Ch. curvisetus, Pseudo-nitzschiapungens, Guianardia delicatula, Rhizosolenia setigera, Proboscia alata f. indica, and Noctilluca scintillans. Phytoplankton cell abundance showed obvious seasonal changes, with the average cell abundance in November, July, and May at 94.79×104 cell/m³, 39.53×104 cell/m³, and 21.5×104 cell/m³, respectively. Phytoplankton diversity index also had obvious seasonal variation. In November, the Shannon-Wiener diversity index, Margalef index, and Pielou index were higher than in May and July. Comparing the artificial reef areas with the control area, the phytoplankton cell abundance of the control in May was significantly higher than the artificial reef areas. In July and November, phytoplankton cell abundance at the artificial reef constructed in 2014 was significantly higher than other artificial reef areas and the control area. The diversity index in both the control and the artificial reef area constructed in 2012 was lower than the 2010 and 2014 reefs, and lowest in the non-artificial reef area. Clearly, the construction of an artificial reef had a significant effect in improving phytoplankton community diversity, but this diversity is not always increasing consistently but fluctuating.