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 the Hulunbuir region, a field experiment was conducted from May to October 2024 in the Tenihe area. The study evaluated the effects of organic fertilizer substitution for chemical fertilizer under nitrogen-equivalent rates at four replacement levels (0%, 25%, 50%, and 75%). Each treatment was arranged in plots of 60 m² with three replicates. Crop yield, 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. 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 contents, indicating improved forage digestibility. In contrast, oat forage yield showed no significant variation among treatments; however, its nutritional quality improved, as the 75% organic fertilizer substitution increased crude protein content by 35.3% compared with the chemical fertilizer treatment. LCA results further indicated that maize with 50% substitution and oat with 75% substitution 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 yield 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.