This study examines the impact of urbanization on wastewater discharge (WD) in 30 provinces in mainland China, considering the inertia characteristics of WD. Various models, including the Tapio decoupling model, dynamic curve relationship model, dynamic threshold effect model, and dynamic quantile model, are employed to analyze the decoupling relationship, curve relationship, threshold relationship, and quantile relationship, respectively. The research findings indicate a shift in the relationship between urbanization and total wastewater discharge (TWD) from expansionary negative decoupling to strong decoupling. Regarding household wastewater discharge (HWD), the relationship is primarily characterized by expansionary negative decoupling and weak decoupling, while industrial wastewater discharge (IWD) is mainly associated with strong decoupling. Urbanization does not exhibit an (inverted) N-shaped relationship with TWD, IWD, and HWD, but it does show an inverted U-shaped relationship with TWD and HWD. The study also reveals that urbanization has a dynamic threshold effect and regional heterogeneity on HWD, but not on TWD and IWD. As the quantile increases, the positive impact of urbanization on TWD and HWD decreases, while the negative impact on IWD increases.