Abstract: Drought has been one of the most harmful abiotic stress to crop growth and development. The growth and physical performance of corn is susceptible to water deficit stress in the middle and late crop stages. Crop straw-derived biochar has been commonly recognized as the soil amendment in agriculture production. The carbon-rich product can be characterized by the loose and porous, high surface area, and oxygen-rich functional groups. The application of biochar in soil can be expected to improve the soil physical and chemical conditions. Better crop physiological performance can also achieve to promote the crop growth and yield. However, it is still unclear on the effects of biochar on photosynthetic fluorescence of corn grown under deficit irrigation in the middle and late stages. Therefore, this study aims to clarify the compensating effect of biochar additions on corn leaf gas exchange and chlorophyll fast fluorescence induction kinetic parameters. The physiological regulation mechanism of corn was also determined to cope with different deficit irrigation regimes. An in-situ test-pit control experiment was carried out in Hohhot, Inner Mongolia, China in May to October in 2023. The 2×3 factorial combinations were set as the biochar rates 0 t/hm² and 30 t/hm² (B) and deficit irrigation regimes (field capacity 95%±5% (well-watered irrigation, WW), 70±5% (moderate water-deficit irrigation, MD), 50%±5% (severe water-deficit irrigation, SD)). A total of six individual treatments (BWW, WW, BMD, MD, BSD, and SD) were then selected. Soil moisture treatment was lasted for 27 days in the heading-filling period. The responsive characteristics and their relationships were determined, including the leaf gas exchange parameters, chlorophyll fast fluorescence induction kinetic parameters, chlorophyll fast fluorescence induction kinetic OJIP curves, abscisic acid and indole acetic acid in corn leaves. The parameters were sampled and then measured at days 3, 13 and 22 after water control. The results showed that the deficit irrigation was induced the synthesis of a large amount of abscisic acid (ABA) in leaves, with the prolongation of water deficit stress time, indicating the decreasing chlorophyll content and limiting photosynthesis. Severe deficit irrigation was resulted in the partial inactivation of photosystem II (PSII), leading to the blocked electron transfer. There was the varying reduction in the fluorescence intensity of I-P phase of chlorophyll fast fluorescence induction kinetic OJIP curves. Compared with the WW, the maximum fluorescence intensity of P phase decreased by 23.55% on average under SD (on the 22nd day of water control), the net photosynthetic rate (P_n), stomatal conductance (g_s), and transpiration rate (T_r) of leaves were also reduced by 42.37%, 41.26% and 34.16%, respectively. In the moderate and severe deficit irrigation, the content of leaf ABA decreased by 6.91%-18.38% and 21.89%-52.15%, respectively, whereas, the content of leaf auxin (IAA) increased by 25.21%-46.75% and 47.77%-82.63%, respectively, under the biochar treatment, compared with non-biochar one. Furthermore, Pn increased by 5.71%-20.64% and 1.66%-32.63%, respectively, when averaged across three samplings. While the I-P phase of chlorophyll fast fluorescence induction kinetic curves was improved significantly with biochar addition. Meanwhile, the maximum quantum efficiency of PSII (F_V/F_M), potential activity of PSII (F_V/F_O), electron transport rate (ψ_o) and photosynthetic performance index (P_IABS) were also improved by biochar addition under deficit irrigation. The application of biochar was improved the efficiency of PSII light energy conversion and electron transfer. The concentration of ABA and IAA in leaves was adjusted to improve the photosynthetic fluorescence performance. The damage of severe water deficit was then alleviated to the chloroplast and PSII of corn. The drought tolerance of corn was enhanced in the middle and late stages. The finding can provide the theoretical and technical support to the carbonization and utilization of straw resources. The regulation mechanism of crop drought resistance can greatly contribute to the efficient utilization of farmland water resources.