In the present study,the seasonal dynamics of photosynthetic activity of Microcystis in Lake Taihu were measured by using Phyto-PAM Phytoplankton Analyzer in the year of 2014. The results show that photosynthetic activity of Microcystis was not detected during winter periods. The maximum quantum yields(F_v /F_m) and the effective quantum yields (DeltaF/F_m ') increased rapidly during spring periods.Two yields(F_v /F_m) and (DeltaF/ Fm ') increased first and then decreased during summer periods,and with a decreasing tendency during autumn periods. The values of F_v /F_m in Meiliang Bay and the center area ranged from 0.34 to 0.55,from 0.28 to 0.50,respectively,with the mean values of 0.42 and 0.39. In parallel,the value of DeltaF/ Fm ' varied from 0.15 to 0.38,and from 0.10 to 0.38,respectively,with the mean values of 0.26 and 0.23. The non-photochemical quenching (NPQ) increased from March to August,and subsequently decreased until to December,with reaching maximum in August. Three paratmeters derived from response light curve(RLC) show an increasing tendency from March to June,and then significantly fluctuated with seasonal change. Correlation analysis between chlorophyll fluorescence parameters and enviromental factors suggest that there were significant positive correlation among F_v /F_m,DeltaF/F_m ' and NPQ,and they all were significantly and positively related to water temperature. Both F_v /F_m and DeltaF/F_m ' were significantly in positive relationship with the maximum electron transport rate(rETR_(max)),and rETR_(max) was positively related with TN. The onset of light saturation (Ik) was significant positively related with TP,but negatively related with TN/TP and NO_3~-.In summery,the seasonal changes for photosynthetic activity of Microcystis was suitable to the dynamics of cyanobacterial bloom formation and development in Lake Taihu,and it was restrained with reducing nitrogen and phosphorus concentrations and controlling global climate warming.