From June to October in 2012, the phytoplankton cell density and some physicochemical factors were investigated in Taiyuan Fenhe impounding regions, involving 8 sampling sites, meanwhile, the relationship between them were analysis. The results indicate that (1) the phytoplankton cell density was relatively high in July and it was diverse at different sampling sites, moreover, the phytoplankton was more in downstream than in upstream; (2) the temperature was relatively high during June to August, declined obviously in the early of September. The variation of DO, CODCr and CODMn showed that water is heavily polluted in August; (3) the phytoplankton cell density in most sampling sites were positively associated with air and water temperature as well as CODCr and CODMn, yet with a negative correlation of DO. At whole, the change of temperature and CODCr had a close relationship with phytoplankton cell density. Aimed at the current water quality of Taiyuan Fenhe impounding regions, some measures should be taken in order to protect water quality and restore ecosystem.