Abstract: Construction activities associated with the Pinglu Canal project, particularly channel straightening and meander cutoff works undertaken during excavation, have substantially altered the groundwater flow field in the surrounding region, thereby exerting a marked influence on regional water use and raising broader concerns regarding water resource sustainability along the canal corridor. To clarify the actual impact of the Pinglu Canal construction on the groundwater system, this study selected a typical reach of the canal as the study area and integrated groundwater level dynamic monitoring with ²²²Rn isotope tracing, employing a radon mass balance model in conjunction with a hydrological flux balance method for mutual validation. This combined methodological approach systematically reveals the disturbance characteristics of the regional groundwater flow field and the intensity of groundwater discharge driven by canal construction, while further elucidating the cascading effects of groundwater level changes on regional water-use structure, thus providing a multi-line evidence framework for understanding engineering-induced hydrological disturbance. The results indicate that the sharp decline in canal water level has established the channel as a new, highly effective regional groundwater discharge base level, leading to a significant drawdown of surrounding groundwater levels and the formation of a stable groundwater flow field directed toward the canal. Seasonal monitoring demonstrates that the discharge effect induced by canal construction persists throughout the observation period; however, with precipitation recharge, groundwater levels generally rebound during the rainy season, highlighting that—even under moderate excavation intensity—the groundwater system along the canal still exhibits pronounced non-steady-state evolution, with the aquifer displaying a certain degree of elastic recovery capacity under seasonal recharge rather than undergoing irreversible depletion. The spatial distribution of ²²²Rn further reveals that groundwater discharges preferentially toward the channel, forming a distinct “radon concentration attenuation zone” that extends continuously along the canal alignment. Radon concentrations increased markedly from upstream to downstream sampling points (from 175 Bq/m³ to 1,904 Bq/m³), indicating intense groundwater inflow within this reach and corroborating the discharge patterns inferred from water level data. Governed jointly by channel excavation depth and the high permeability characteristics of the karst aquifer, the ²²²Rn mass balance model estimates a groundwater discharge flux into the canal of 291.2–344.5 m³/(d·m), a result further corroborated by the independently derived hydrological flux balance estimate (311.0–380.2 m³/(d·m)), thereby confirming the reliability and consistency of this discharge estimate across methods. Moreover, the reduction in groundwater storage caused by channel excavation has driven a shift in the regional water supply pattern from dispersed shallow extraction toward centralized deep groundwater extraction, further promoting a partial transition in agricultural planting structure from paddy fields to dryland crops better adapted to the altered hydrological regime. Compared with 2021, the rice cultivation area in 2024 decreased by 78.2%, while the maize cultivation area increased by 861.8%. Considering multiple contributing factors, such as market conditions, policy adjustments, and climate variability, and comparing with surrounding control areas with similar topography and agricultural production conditions that remained unaffected by the canal project during the same period, this analysis further confirms that the decline in shallow groundwater levels and the reduced irrigation water supply capacity—both directly induced by canal excavation—constitute the key controlling factors driving the substantial restructuring of planting patterns observed in the study area. In summary, this study elucidates the cascading response chain linking construction-induced groundwater flow field changes to adaptive shifts in societal water-use structure during major canal construction, offering reference value for water resource management and ecological risk prevention and control in similar large-scale engineering contexts. Future research incorporating long-term coupled monitoring and socio-hydrological coupled simulation could further reveal the long-term evolution patterns of surface–groundwater interactions under sustained anthropogenic disturbance.