Maize (Zea mays L.) is a global staple crop for food, feed and industrial use in China. As a thermophilic crop, its seedlings suffer from cold stress. Heilongjiang, China’s major maize producer, faces worsening early cold that impedes germination and seedling growth, causing heavy yield losses. Improving germination cold tolerance stabilizes yields. Ubiquitous proline (Pro) works as a key osmoprotectant and core stress-resistance regulator in plants. In this field experiment, the maize cultivar Damin 3307 was used to explore the effects and regulatory mechanisms of Pro seed soaking on low-temperature-stressed maize. We analyzed seedling emergence, plant growth, photosynthetic performance, nitrogen metabolism, dry matter accumulation, ear traits and grain yield of treated maize under low temperature. The results showed that seed soaking with 15 mmol·L⁻¹ Pro effectively improved the seedling emergence rate of early-sown maize under low temperature and compensated for cold-induced population decline. It significantly increased plant height at the seedling and jointing stages, which enhanced vegetative growth. Exogenous Pro regulated the activities of key nitrogen metabolism enzymes, including NR, GS, GOGAT, GDH, GOT and GPT, alleviating low-temperature-induced nitrogen metabolism inhibition and accelerating ammonia assimilation as well as protein and amino acid synthesis to promote maize growth. Additionally, Pro soaking mitigated the cold-induced suppression of leaf photosynthetic indices (Pn, Gs, Tr, Ci) and key photosynthetic enzymes (RuBPCase, PEPCase). It maintained higher leaf chlorophyll content and green leaf area, delayed leaf senescence, and sustained stable carbon and energy supply for nitrogen metabolism. These improvements strengthened maize cold resistance, promoted dry matter accumulation, optimized ear and kernel traits by increasing ear number and kernels per ear and reducing empty kernels, and ultimately improved the grain yield of early-sown maize under low-temperature stress.