Crop straw accumulation emerged as a pressing environmental challenge, particularly in semi-arid and alkaline agricultural regions where low moisture, high soil pH, and unstable abiotic conditions constrain microbial degradation efficiency. In this study, a semi-arid and alkaline-tolerant lignocellulose-degrading fungal strain, Porostereum spadiceum Y-1, was isolated from farmland soils in northern China and systematically characterized. Enzymatic assays confirmed that P. spadiceum Y-1 secretes a synergistic cellulolytic enzyme system, which collectively mediates substantial lignocellulose degradation. Response-surface methodology was applied to optimize fermentation conditions, leading to a significant enhancement of enzyme activity. Carrier screening experiments identified corn stover as a cost-effective and environmentally friendly inoculum matrix. Pot experiments conducted in alkaline soil demonstrated that P. spadiceum Y-1 enhanced organic matter turnover, promoted nutrient release, and accelerated straw decomposition. This study fills a key research gap by quantitatively evaluating the performance of a fungal strain specifically adapted to semi-arid alkaline soils, and provides evidence for its potential application in sustainable straw-return systems.