Abstract: Coffee grounds, a major by-product generated during the production of instant coffee, are rich in galactomannan. Mannanase can hydrolyze this compound into mannose and manno-oligosaccharides with various functional activities, thereby effectively increasing the added value of coffee grounds. In this study, the β-mannanase gene (AnMan26) from Aspergillus niger was recombinantly expressed in the engineered A. niger strain FBL-B. The enzymatic properties of the enzyme were characterized, and it was then employed to hydrolyze coffee grounds for the production of mannose and manno-oligosaccharides. The results showed that the recombinant A. niger produced mannanase at an expression level of 1830 U/mL after high-density fermentation for 168 h. The optimum pH of the enzyme was 5.0 and the optimum temperature was 45 ℃. The enzyme remained stable within a pH range of 3.0-7.0 and at temperatures below 45 ℃. A combination of AnMan26 and α-galactosidase were used to hydrolyze the microwave-treated coffee grounds. After 8 h of hydrolysis, the hydrolysis rate of galactomannan in the coffee grounds reached 42.3%, with a hydrolysate yield of 20.3% coffee grounds. The main components of the hydrolysate were mannose (44.1%), mannobiose (2.5%), mannotriose (10.9%), and mannohexaose (1.5%). The excellent properties of AnMan26 enable the effective hydrolysis of coffee grounds to produce mannose and manno-oligosaccharides, providing both theoretical and practical basis for the high-value utilization of coffee grounds.