Abstract: Maize (Zea mays) and oat (Avena sativa) are the major forage crops cultivated in northern China, providing essential feed resources for the regional livestock industry and supporting local economic development. To optimize fertilization strategies and promote sustainable forage production in Hulunbuir, a field experiment was conducted from May to October 2024 in the Tenihe area. The study evaluated the effects of organic fertilizer substitution at four levels (0%, 25%, 50%, and 75%) for chemical fertilizer under equivalent nitrogen. Each treatment was arranged in plots of 60 m² with three replicates. Crop yields, forage nutritional quality, and environmental impacts were systematically assessed, with the environmental performance analyzed through life cycle assessment (LCA) considering the full agricultural production process as the system boundary. The results demonstrated that partial substitution of chemical fertilizer with organic fertilizer significantly influenced maize growth and productivity. The whole-plant maize yield under 50% organic fertilizer substitution was 28.9% higher than that under full chemical fertilization, accompanied by notable decreases in cellulose and acid detergent fiber content, indicating improved forage digestibility. Oat forage yields showed no significant variation among treatments. However, the nutritional quality of oat forage improved, as the 75% organic fertilizer substitution increased the crude protein content by 35.3% compared with full chemical fertilization. LCA results further indicated that the treatments of 50% organic fertilizer substitution for maize and 75% organic fertilizer substitution for oat achieved lower overall environmental impacts, particularly in terms of energy consumption and greenhouse gas emissions. A comprehensive evaluation using a membership function model revealed that 50% organic fertilizer substitution was the optimal fertilization mode for maize (membership value = 0.539) and for oat (0.626), balancing agronomic performance and environmental sustainability. These findings highlight that partial replacement of chemical fertilizer with organic fertilizer can enhance forage yields and nutritional quality while mitigating environmental burdens associated with agricultural production. This study provides theoretical guidance and practical evidence for establishing an eco-friendly and resource-efficient forage production system in northern China, offering a valuable reference for regional fertilizer management and sustainable agricultural development.