Abstract: A phytoplankton survey was carried out in Beihai inshore waters in the spring of 2008 and 2009 to analyze the species composition of phytoplankton and monthly change of dominant species.The results show that in 2008, there are 3 phyla, 37 genera and 71 species of phytoplankton, among which 26 genera and 50 species are the predominant species, Bacillariophyta. Additionally, 10 genera and 20 species are Pyrrophyta, with 1 genus and 1 species Chromophyta, and the average monthly quantity of phytoplankton is 13.65×10³ ind · L^-1. As for 2009, there are 3 phyla, 39 genera and 72 species of phytoplankton, among which 33 genera and 58 species are the predominant species, Bacillariophyta. Besides, 5 genera and 13 species are Pyrrophyta, with 1 genus and 1 species Chromophyta, and the average monthly quantity of phytoplankton is 14.07×10³ ind · L^-1. We observed obvious variation in the dominant species and quantity distribution of phytoplankton.