To clarify the effects of biochar application on leaf photosynthesis, dry matter accumulation, and productivity in a maize–soybean intercropping system, a two-year field experiment was conducted in the Yellow River irrigation area of Inner Mongolia from 2024 to 2025. A split-plot design was adopted with two biochar application rates (0 and 5 t ha⁻¹) and three cropping patterns, including maize monoculture, soybean monoculture, and maize–soybean 2:4 intercropping. Leaf SPAD values, photosynthetic characteristics (Pn, Tr, Gs, and Ci), yield components, and land equivalent ratio (LER) were determined. Compared with maize monoculture, intercropping significantly increased maize SPAD values at the V12 and VT stages by 12.80% and 13.39% in 2024 and by 15.41% and 20.58% in 2025, respectively, and enhanced maize Pn, Tr, and Gs at the V12 and R1 stages. Soybean showed greater sensitivity to intercropping, with reduced SPAD values, Pn, Tr, and Gs during the branching, flowering, and pod-setting stages, whereas biochar application partially alleviated these inhibitory effects. Intercropping increased maize kernel number per ear and thousand-kernel weight but reduced soybean effective plant density, grain number per plant, and grain yield. Biochar application improved the grain yield of both intercropped maize and soybean. Under biochar application, the LER values reached 1.04 in 2024 and 1.21 in 2025, indicating a clear advantage in land-use efficiency. Overall, biochar application and maize–soybean intercropping were associated with improved photosynthetic performance, higher land-use efficiency, and increased system productivity.