A parent strain Aspergillus niger LW-1 was mutated by the compound mutagenesis of vacuum microwave (VMW) and ethyl methane sulfonate (EMS). A mutant strain, designated as A. niger E-30, with high- and stable-yield β-mannanase was obtained through a series of screening. The β-mannanase activity of the mutant strain E-30, cultivated on the basic fermentation medium at 32°C for 96h, reached 36,675U/g dried koji, being 1.98-fold higher than that (18,501U/g dried koji) of the parent strain LW-1. The purified E-30 β-mannanase, a glycoprotein with a carbohydrate content of 19.6%, had an apparent molecular weight of about 42.0kDa by SDS–PAGE. Its optimal pH and temperature were 3.5 and 65°C, respectively. It was highly stable at a pH range of 3.5–7.0 and at a temperature of 60°C and below. The kinetic parameters K m and V max , toward locust bean gum and at pH 4.8 and 50°C, were 3.68mg/mL and 1067.5U/mg, respectively. The β-mannanase activity was not significantly affected by an array of metal ions and EDTA, but strongly inhibited by Ag + and Hg 2+ . In addition, the hydrolytic conditions of konjak glucomannan using the purified E-30 β-mannanase were optimized as follows: konjak gum solution 240g/L (dissolved in deionized water), hydrolytic temperature 50°C, β-mannanase dosage 120U/g konjak gum, and hydrolytic time 8h.