Abstract: Combined salt and drought stress poses a more pronounced constraint on crop productivity than either stress alone. While biochar has been shown to effectively mitigate the adverse effects, a systematic understanding of its regulatory mechanisms remains elusive due to the heterogeneity in soil type, crop cultivars and biochar properties. This study therefore conducted a meta−analysis synthesizing 19 publications containing 227 observations (150 for yield, 77 for water use efficiency) to scrutinize biochar effects on yield and water use efficiency. Random meta−forest model was employed to identify the key factors of biochar on yield and water use efficiency. Additionally, four−quadrant analysis was performed using 62 paired observations containing both yield and water use efficiency to further explore the synergistic responses to biochar application. Results showed that the application of biochar increased crop yield by 5.6% (95% confidence interval, 2.9%−8.4%) and water use efficiency by 6.2% (2.7%−9.8%). These promoting effects were more pronounced under pot experiments, C₄ crops, soils with electrical conductivity (EC) < 2 dS/m and pH 7.5～8.5, as well as mild to moderate combined salt–drought stress. Moreover, distinct responses emerged between yield and water use efficiency depending on biochar properties. Specifically, woody−derived or high−temperature (≥600℃) biochar exhibited stronger yield promotion, whereas straw−derived or low−temperature (≤400℃) biochar demonstrated greater enhancement of water use efficiency. AIC (Akaike information criterion) weight analysis further revealed that the application rate, pyrolysis temperature, salt–drought stressed type, soil type, soil pH, and crop type were the dominant modulators of yield, whereas considerably fewer factors carried high importance for water use efficiency. Notably, biochar induced concurrent increases in both yield and water use efficiency in 74.19% of the 62 paired observations, revealing a synergistic “win–win” effect on yield enhancement and water conservation. However, this synergistic benefit was substantially weakened under high−intensity combined salt–drought stress. Collectively, these findings highlight the potential of biochar as a promising amendment to concurrently improve crop productivity and water conservation in agricultural systems subjected to combined salt–drought stress.