This study systematically investigated the effects of Lactiplantibacillus plantarum (Lp), cellulase (CE), and their combination (CELp) on the fermentation quality, fiber degradation, and microbial community in rapeseed straw silage. This research was carried out with a completely randomized design, and rapeseed straw was inoculated with L. plantarum (1.0 × 10⁶ colony-forming units [CFU]/g), CE (50,000 U/g), or CELp and fermented in vacuum-sealed bags at 25°C for 60 days. Results demonstrated that the CELp co-treatment yielded superior outcomes, significantly increasing lactic acid content, accelerating acidification, and effectively degrading neutral detergent fiber, acid detergent fiber, and hemicellulose compared to individual treatments. Microbial community analysis demonstrated that the CELp co-treatment effectively enriched functional bacterial treatments represented by Lactobacillus and Xanthomonas while significantly enhancing the activities of core enzymes, including L-lactate dehydrogenase, xylanase, and β-glucosidase. The CELp co-treatment intensified homolactic fermentation and structural carbohydrate decomposition, with correlation analysis revealing tight links among microbial composition, enzyme activities, and silage quality. In conclusion, the microbial–enzyme synergistic approach effectively improved the fermentation quality of rapeseed straw silage by directionally modulating the microbial community structure and enhancing core enzyme activities. These findings provide a novel theoretical basis for the efficient utilization of lignocellulosic agricultural by-products.