Nitrogen (N) fertilizer regulates leaf photosynthesis and dry matter production in rice, and extensive genotypic differences in N responsiveness exist among cold-region japonica rice cultivars. A two-year field experiment was conducted in the city of Jiamusi, Heilongjiang Province, during 2023–2024, using two representative japonica cultivars: the small-panicle cultivar Longjing 47 (LJ47) and the large-panicle cultivar Longjing 3010 (LJ3010). A split-plot design was adopted with four N application rates (0, 110, 138, and 166 kg N·ha⁻¹) and four basal–tiller to panicle–grain N ratios (10:0, 8:2, 7:3, and 6:4). The results showed that increasing N supply significantly enhanced SPAD values, photosynthetic capacity, leaf area index, and total dry matter accumulation in both cultivars. LJ47 maintained high photosynthetic capacity under moderate N supply, whereas LJ3010 required higher N input to sustain superior photosynthetic performance. From heading to maturity, dry matter accumulation, photosynthetic potential, crop growth rate, net assimilation rate, N uptake, and grain yield all increased with an increasing ratio of panicle–grain N fertilizer, with more pronounced improvements observed in LJ3010. LJ47 achieved optimal physiological performance and grain yield at 138 kg N·ha⁻¹ (moderate N), with no further promotion under high-N supply. In contrast, LJ3010 required 166 kg N·ha⁻¹ (high-N) to maintain photosynthetic advantages and achieve higher grain yield in the late growth stage. Increasing the proportion of basal–tiller N fertilizer favored the source sink balance of LJ47, while increasing panicle–grain topdressing delayed leaf senescence and promoted post-heading dry matter translocation in LJ3010. For a target grain yield of 9 t·ha⁻¹, the recommended N rate is 138 kg·ha⁻¹ with a basal–tiller to panicle–grain N ratio of 8:2–7:3 for LJ47, and 166 kg·ha⁻¹ with a ratio of 7:3–6:4 for LJ3010. This study provides a theoretical basis and data reference for the formulation of precise N fertilization strategies that match fertilizer management with rice cultivar characteristics in cold regions.