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In the southern Huang-Huai-Hai (HHH) region, China, maize production is frequently threatened by waterlogging at the seedling stage and by drought at the big flare stage. A two-year field experiment was performed to explore whether subsoiling (SS) in the winter wheat season could improve the photosynthetic capacity and increase the water use efficiency (WUE) of summer maize using the variety, Luyu9105. A split design was adopted in the experiment. The main plots used tillage practices, including SS and rotary tillage (RT). The subplots consisted of two irrigation methods, i.e., applied supplemental irrigation at the big flare stage (I) and no irrigation at the big flare stage (NI). The results showed that the SS treatment significantly increased soil water content (SWC) in the 40–60 cm soil layer. The SS treatment improved green leaf area index (gLAI) by 15.1%–30.2%, and enhanced the ear-leaf net photosynthetic rate (Pn), stomatal conductance (gs), intercellular CO2 concentration (ci) and transpiration rate (Tr), and was accompanied by an increase in the grain-filling duration (T) by 5 days and the mean grain-filling rate (Va). The SS treatment decreased the stomatal limitation (Ls), indicating that RT treatment, which was under lower SWC, led to a decrease in Pn. Applied supplemental irrigation under RT treatment was able to compensate for the growth of leaves, but could not reverse the decreasing trend in the gLAI. Ultimately, the SS treatment improved WUE by 9.1%–9.9%, and increased grain yields by 10.0%–29.3%. Therefore, this study showed that in the southern Huang-Huai-Hai Plain, which has a yellow cinnamon soil type, the practice of SS can improve the photosynthetic characteristics of leaves and WUE of rainfed summer maize. [ABSTRACT FROM AUTHOR]