Small hydropower stations (SHS) not only play an important role in electricity generation and water resource regulation in mountain areas, but also at the same time, impose ecological and environmental impacts. To study the responses of phytoplankton community in rivers with construction of SHS, samples of phytoplankton from 7 rivers in Hainan Province where SHS had been or will be built were taken and analyzed. According the results, a total of 178 phytoplankton species belonging to 62 genera were identified, and the dominant species includes Achnanthaceae, Gomphonema, Nitzschia, Melosira granulat, Phormidium, Oscillatoria, Chlorella vulgaris, Merismopedia, and Navicula. Phytoplankton abundance in rivers was from 5.1×10⁴cells/L to 63.6×10⁴cells/L, the diversity index was from 2.73 to 4.53, and Desmids had been found in most SHS. SHS had a great influence on phytoplankton biomass, species composition and dominant species in the rivers. Comparing with thfe rivers with SHS, the rivers without SHS had higher Shannon-Wiener diversity index and the composition of phytoplankton species were more balanced, Cyanobacterias and some Diatoms always dominated the phytoplankton community in the rivers, such as in NDJ-JJ-9 (Cyanophyta 25.7%,Chlorophyta36.9%, Bacillariophyta 36.1%), ZBJ-MMD-11 (Cyanophyta 41.8%, Chlorophyta 33.2%,Bacillariophyta 23.1%), and WLH-SX-13(Cyanophyta 29.1%, Chlorophyta 25.2%, Bacillariophyta 38.8%). In the rivers with SHS, large diatom Achnanthacea,Gomphonema and Nitzschia becoming predominated were common, such as in CHJ-YJ-1 (Bacillariophyta 91.3%), CHJ-YJ-2 (Bacillariophyta 65.9%), and NDJ-GL-6 (Bacillariophyta 56.5%).To phytoplankton abundance, most rivers without SHS were 20-30×10⁴cells/L, while the rivers with SHS were widely different 5-160×10⁴cells/L. The rivers with SHS had higher average abundance and abundance difference index than the ones without SHS. The construction of SHS had changed the hydrodynamic of the rivers, and the changed hydrodynamic might had played an important role in the phytoplankton community. In mountain areas of Hainan province, most rivers were Shallow with less water flow. Stable water environment was suitable for Cyanobacterias and some Diatoms in the river without any SHS. Once SHS had been built, agitation of water body increased and lager Diatoms such as Achnanthaceae Gomphonema and Nitzschia might be dominated. The construction of SHS would also promoted the biomass of the river, but weaken the stability and proportionality of phytoplankton community by increasing fluctuation and agitation. Among cascade SHS, the barrier of the stations, the phytoplankton community could still be some similar especially between adjacent SHS. The correlation coefficient of phytoplankton community between adjacent SHS were all over 0.9 with highly significant correlation(P<0.01).