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Due to limited freshwater availability for winter wheat and summer maize, grain production in the annual double-cropping system of the low plain surrounding the Bohai Sea in North China is strongly influenced by inter-annual rainfall variability. The relatively abundant saline water resources in this region offer a potential source for irrigation. This study aimed to evaluate the effects of additional saline water irrigation under deficit irrigation on the crop yields and water productivity of winter wheat and its following crop maize, as well as to determine the soil salinity dynamics and annual salt balance under saline irrigation. A two-year field experiment (2023–2025) was conducted using six irrigation treatments, namely rainfed (I0), one freshwater irrigation (If), one saline irrigation (Is), combinations of freshwater and saline irrigation (Is + If, If + Is), and two freshwater applications (If2) to evaluate the effects of an additional saline water irrigation event, compared with the commonly used freshwater irrigation regime, on crop yields, water productivity, and the soil salt balance. The results showed that a single saline irrigation event (70 mm) increased the wheat yield by 18–38% under rainfed conditions and by 7–10% under limited freshwater irrigation. In contrast, the maize yield was not affected by the additional saline irrigation applied during the winter wheat season. Although salt accumulation occurred in the topsoil following the saline irrigation of winter wheat, it did not impair maize growth, owing to salt leaching during irrigation for maize emergence and concentrated summer rainfall. Within the two-year observation period, no progressive salt accumulation was observed in the top 1 m soil profile. These findings indicate that the strategic use of saline water to supplement the crop water supply can enhance crop production under deficit irrigation, provided that soil salinity is effectively managed.