This study developed a compound enzyme-assisted microbially fermented concentrate for Yanbian steers. The enzyme formulation was optimized using response surface methodology, with acid-soluble protein/crude protein ratio (ASP/CP), ammonia-N/total N ratio (AN/TN), and pH as response variables. The optimized enzyme preparation contained 619.15 U/g protease, 742.08 U/g amylase, and 257.75 U/g β-mannanase. The measured ASP/CP, AN/TN, and pH values were 31.74%, 3.56%, and 4.32, respectively, closely agreeing with the predicted values of 32.00%, 3.55%, and 4.28. Membership function analysis, which normalized positive and negative indicators to a common 0–1 scale and integrated them into a comprehensive score, supported the selection of day 5 as the most balanced fermentation duration under the primary equal-indicator weighting scheme. Thirty Yanbian steers were randomly assigned to receive microbially fermented concentrate without compound enzyme addition (MF) or compound enzyme-assisted microbially fermented concentrate (MEF) at 500 g/head/day on an as-fed basis for 100 days. After adjustment for initial body weight using ANCOVA, MEF increased final body weight by 3.9% and average daily gain by 35.7% compared with MF (both p < 0.001). MEF also increased dry matter intake by 12.1% (p = 0.018) and reduced feed conversion ratio by 18.4% (p = 0.008). MEF increased dry matter, crude protein, neutral detergent fiber, and acid detergent fiber digestibility by 4.3%, 5.4%, 22.6%, and 15.6%, respectively, and reduced blood urea nitrogen by 18.9%. MEF also increased ruminal total volatile fatty acids and decreased ruminal NH₃-N and the acetate-to-propionate ratio (p < 0.05). Observed ASVs, Chao1, and Shannon indices did not differ between treatments, whereas PERMANOVA detected a significant difference in overall rumen bacterial community structure (R² = 0.180, p = 0.023). VIF-based redundancy analysis indicated that variation in rumen bacterial community structure were correlated with crude protein digestibility, total volatile fatty acids, and blood urea nitrogen. Overall, within the microbial-fermentation system evaluated in this study, compound enzyme addition increased growth performance and apparent nutrient digestibility, reduced ruminal NH₃-N and blood urea nitrogen concentrations, and improved feed efficiency in Yanbian steers.