Understanding the relationship between human activities and ecological environment and identifying the changing areas of the ecological environment and their causes is of great significance to the formation of ecological protection policies for differentiated regions. The Taihu Lake basin is the core of the Yangtze River Delta, which has experienced rapid economic growth (GDP +15.7%/year), population growth (+3.0%/year on average), urbanization (+ 9.2%/year on average) from 1985 to 2010. Rapid industrialization and urbanization have significantly changed the pattern of land use, which has posed a threat to the region's ecological security and sustainable development. Based on the MODIS 17A3 and DMSP/OLS nighttime light images, we determined the urban development level index, and a linear trend analysis was used to partition the urban development levels. We used the Pearson correlation coefficient to calculate the relationship between the urban development level and net primary productivity (NPP). Finally, a spatial correlation model was used to analyze the impact of urban expansion on the NPP in the Taihu Lake basin, combined with geographic information system (GIS) and remote sensing (RS) techniques. The results showed that the average NPP in the Taihu Lake basin varied from 388.79 gC m^-2·a^-1 to 452.54 gC m^-2·a^-1 from 2000 to 2010. The average NPP over 11 years was 422.65 gC m^-2·a^-1. The changes in average annual NPP were fluctuating and declining. The urban development level slow-increased zone had little effect on the change in NPP, while the increased zone and rapid-increased zone had great influence on the NPP. Along with the increase in the urban development level, land transformation mainly occurred from farmland to construction, forest to construction, and water to construction. The rapid expansion of urban areas has led to increase in construction land and the urban development level, which are the main reasons for the decrease in NPP.