The underwater light distribution is an important factor affecting aquatic ecosystems. The study on the attenuation characteristics of photosynthetically active radiation is of guiding significance for the restoration of submerged plants. In this study, based on the biomass data of submerged plants, eastern Lake Taihu was divided into submerged plants sparse region, submerged plants lush region, and no-plant region. Spatial difference in light attenuation characteristics and light attenuation factors of eastern Lake Taihu, dominant attenuation factors in different regions, regime shift phases and eutrophication level, and the relationship between euphotic depth and transparency were studied by applying in situ measurement data in August 2019. Results were analyzed to provide relevant optical data for the restoration and protection of submerged plants in eastern Lake Taihu. The results showed that the light attenuation characteristics are significantly different in three regions. The diffuse attenuation of photosynthetically active radiation varied from 0.73 m^-1 to 11.80 m^-1 and the corresponding euphotic depth was 0.39-6.31 m. There were significant differences in the concentrations of suspended particulate inorganic matter and suspended particulate organic matter in different regions. The concentration of chlorophyll-a in the sparse region was significantly higher than that in the lush region, but not significantly different from that in no-plant region. Chromophoric dissolved organic matter (CDOM) absorption coefficient had no significant difference in three regions. The linear fitting effect of k_d(PAR) with suspended particulate inorganic matter and suspended particulate organic matter was better, but it was worse with chlorophyll-a and CDOM. Water absorption and scattering were the main cause of light attenuation in the lush region and suspended particulate inorganic matter was the dominant attenuation factor in no-plant region. The sparse region was dominated by chlorophyll-a and suspended particulate inorganic matter, which was a region that needs to be focused on for ecological restoration. Suspended particulate organic matter and CDOM had little effect on the light attenuation. Eastern Lake Taihu was currently in the transition stage from a macrophytes-dominated state to a phytoplankton-dominated state, and the entire lake was at eutrophic level. Euphotic depth was about 2.7 times of the transparency.