Investigating the coordinated development between social economy and water environmental quality is an important basis for promoting the protection of water ecological environment. In this paper, a comprehensive index system was built to estimate the coupling coordination between social economy and water environmental quality subsystems in Nansi Lake catchment, China. Improved coupling coordination degree (CCD) model, scissors difference and projection pursuit model based on genetic algorithm were developed to investigate the spatiotemporal distribution and dynamic evolution mechanism of coupling coordination in social economy–water environment system. Results showed that: (1) Comprehensive level of two subsystems showed an overall uptrend, water environmental quality in the catchment was improved gradually with socio-economic development. (2) The status of coupling coordination between two subsystems had transformed from moderately unbalanced development (CCD of 0.265 in 2001) to favorably balanced development (CCD of 0.667 in 2017). The relatively low CCD in Heze generated the biggest restriction on the coordinated development between two subsystems, compared to Zaozhuang and Jining. (3) The speed difference between two subsystems showed a downward trend, the scissor difference was controlled effectively. And the social economy–water environmental quality system gradually tended to the coordinated development. Overall, the improved CCD model and scissors difference analysis in this paper may provide a new approach for empirical research on environment–economy nexus. We hope this study can help decision makers formulate measures to promote the coordinated development between social economy and water environmental quality subsystems.