Abstract: Drought has posed a serious threat to the many physiological and growth functions of crops. Brassinolide (BR) can be expected to mitigate the drought stress in crops during production. However, it remains unclear whether the BR can consistently enhance crop yield and water productivity under both full irrigation (FI) and deficit irrigation (DI). It is also lacking in the physiological mechanisms underlying the maize response to the BR. In this study, BR was used to modulate the hydraulic transport and abscisic acid for maize production under drought. Three assumptions were that: 1) The foliar application of BR enhanced the maize yield and water productivity under both FI and DI conditions; 2) The BR improved the stomatal conductance and photosynthetic rate by co-regulating hydraulic and abscisic acid signalling, thereby increasing the maize yield and productivity. A field experiment was conducted in the arid region of Northwest China in 2023. Two treatments (control and BR) were performed on the leaf exogenous substance. Three maize genotypes (MC670, Zhengdan 958, and Zhongdan 2) were selected under two modes of irrigation (FI and DI). The irrigation volume of FI treatment was predicted as the crop evapotranspiration. While the irrigation amount for DI treatment was 50% of the total for the FI treatment during the entire growth stage. The BR treatment involved the foliar spraying of 0.1 μmol/L 28-homobrassinolide solution twice at 15-day intervals after the flowering stage. The control treatment was sprayed with distilled water. The leaf's physiological and biochemical parameters were measured three days after BR treatment. The maize yield, crop evapotranspiration, and water productivity were collected at harvest. A systematic investigation was made on the differences in the physiological response mechanisms of the maize yield and water productivity on the BR under FI and DI conditions. Analysis of variance (ANOVA) was also conducted to compare the effects of BR on these indexes. The correlation analysis and Mantel's test were used to explore the physiological mechanism of the BR on the maize yield and water productivity under drought. It was found that the BR significantly increased the maize production under DI treatment. There was no significant effect on the gas exchange, chlorophyll fluorescence, hydraulic indexes, endogenous hormone, SPAD, crop evapotranspiration, leaf area index, biomass, yield and its components, or water productivity under FI treatment. Specifically, the BR increased the kernel number, biomass, net photosynthetic rate, stomatal conductance, midday leaf water potential, whole-plant hydraulic conductivity, SPAD, and reduced leaf abscisic acid content. However, the BR shared no effect on the crop evapotranspiration, leaf area index, chlorophyll fluorescence parameters, predawn leaf water potential, or indoleacetic acid under DI treatment. The variations in the yield and irrigation water productivity were significantly correlated with the kernel number, photosynthetic rate, stomatal conductance, predawn leaf water potential, midday leaf water potential, whole-plant hydraulic conductance, and abscisic acid. The yield gains were induced by BR under DI treatment. The following factors were attributed to: 1) an increase in the whole-plant hydraulic conductance and leaf water potential; 2) a reduction in the leaf abscisic acid content; 3) the combined effects of 1) and 2) on the stomatal conductance; and 4) the subsequent rise in both stomatal conductance and SPAD. The photosynthetic rate was improved to the kernel number. The high yield was dominated by the crop and irrigation water productivity induced by BR under DI treatment, rather than the crop water use. The second hypothesis was confirmed, rather than the first one. This finding can provide insights into the physiological mechanisms of drought stress alleviation in crops using BR. Crop productivity can be expected to improve in arid regions. Therefore, the BR can be combined with water-saving irrigation for high crop yields and efficient water use in the future. The findings can also provide support for sustainable agriculture.