The Sanjiang Plain (SJP), a major black soil region in Northeastern China, has undergone extensive conversion of drylands to paddy fields since the 2000s. This shift has significantly altered surface-atmosphere water-heat exchanges, as well as groundwater consumption and recharge rates. The balance between groundwater consumption and paddy expansion remains a critical challenge in regional sustainable development. This study developed an integrated downscaling framework based on physically consistent model data to downscale Gravity Recovery and Climate Experiment (GRACE) observations, thereby generating high-resolution estimates of groundwater storage anomaly (GWSA) across the SJP. Impacts of climate change and agricultural pattern shift on GWSA variations were assessed, with particular emphasis on their spatial linkages. Findings indicated regional groundwater storage and sustainability experienced an overall decline followed by a marked recovery. Partial Least Squares Structural Equation Modeling identified shifts in regional dry-wet conditions as the dominant driver of GWSA dynamics. However, cropping pattern shift exerted increasingly negative impacts on GWSA at spatial scale, dominating 33.6 % of the agricultural grids. This influence was particularly pronounced in the eastern region, where paddy expansion drove sustained deterioration in groundwater sustainability. Consequently, to address this spatial mismatch, moderating the pace of paddy expansion and adopting water-saving irrigation techniques in northeastern region are recommended. Scenario simulations indicated a sustainable groundwater balance (GWSA slope >0) could be maintained if maize-to-rice conversion remains below 1700 +/- 700 km(2). Through solution-oriented research integrating GRACE and hydrological modeling, this study provides novel insights for balancing regional food production goals with sustainable groundwater management.