Faba bean (Vicia faba L.) is a vital cool-season food legume in China, but its production is often limited by drought stress, particularly in the rainfed regions of Northwest China. The grain yield of faba bean is strongly affected by environmental conditions. Therefore, multi-environment trials are essential for identifying widely adaptable, high-yielding spring faba bean genotypes. This study assessed genotype-by-environment interaction (GEI) in spring faba bean across six rainfed sites from 2018 to 2023 (36 environment-year combinations). Linear mixed models (LMM), weighted average absolute scores of BLUPs (WAASB), multi-trait stability index (MTSI), partial least squares (PLS) regression and factorial regression were used to identify stable, high-yielding genotypes and clarify key rainfall constraints underlying water-limited grain yield gaps in Northwest China. Grain yield (GY) ranged significantly across environments, ranging from 4292.71 to 5474.60 kg ha(-)& sup1; . Decision Support System for Agrotechnology Transfer (DSSAT) simulations showed considerable water-limited grain yield gaps between water-limited potential grain yield and actual grain yield, with the largest gaps observed in Jishishan County. GEI significantly affected all traits (p < 0.05) except days to flowering and days to maturity. Based on WAASB and MTSI, genotypes G18, G12, G11, G17, and G20 showed high grain yield, good stability, and relatively high harvest index (HI). Notably, G11 and G20 recorded the highest HI (44%), representing a 12.82% increase compared with the control. A highly significant negative correlation was found between HI and AGB (p < 0.001), indicating a source-sink tradeoff between vegetative growth and grain assimilate partitioning. PLS regression and factorial regression indicated that rainfall in April was critical for seedling establishment, while rainfall in June and July was essential for pollination and pod-filling stages. This study highlights the importance of multi-environment trials for identifying stable, high-yielding spring faba bean genotypes and determining key monthly rainfall constraints at critical growth stages. The findings provide practical guidance for breeding drought-adapted varieties and enhancing the sustainability of faba bean production in the arid and semi-arid regions of Northwest China.