Energy conversion efficiency of photoautotrophic microalgae plays an important role in the utilization of light energy for cell growth and production of metabolites. To understand the utilization of light energy, Synechococcus sp. PCC7942 was cultivated at different incident light intensities of 15.8, 47.3, and 94.6 ${mu}mol/m^2/sec$ in continuous culture. The influence of light on the carbon and energy metabolism of microalgae was investigated by combining metabolic flux analysis (MFA) and chlorophyll fluorescence analysis (CFA). Results showed that the yields of biomass based on ATP ($Y_{ATP}$) and total light energy ($Y_E$) both declined with increasing light, and the maximal values of $Y_{ATP}$ and $Y_E$ were estimated to be 4.73 g/mol-ATP, and $17.10{ imes}10^{-3}$ g/kJ respectively, at the examined conditions. The overall efficiency of energy conversion against total absorbed energy changed with the varying irradiances. However, the actual conversion efficiency of total energy based on CFA was almost constant, regardless of the different irradiances used in the present study.