To improve β-mannanase activity, the molecular evolution of β-mannanase gene from Bacillus licheniformis KD-1 was carried out by error-prone PCR. The mutated genes were expressed in B. subtilis to directed screen the mutants with higher β-mannanase activity than the wildtype. Among mutants, the specific activity of β-mannanase ManBl (I91N/L211I) was 15554.7 U/mg, which was 4.2 times of the wild-type ManBl. The extracellular yield of ManBl (I91N/L211I) expressed in B. subtilis at food-grade level reached 17601.3 U/mL. The β-mannanase structure predicted by AlphaFold2 demonstrates that the two loci (I91N and L211I), which are not the β-mannanase catalytic center, influence the enzyme activity at a great degree. Furthermore, the β-mannanase ManBl (I91N/L211I) can hydrolyze konjac glucomannan (KGM) into mannooligosaccharides (MOS) with different degree of polymerization (DP), which were mainly composed of mannohexaose, mannotriose, and mannobiose. This is the first report that the combined mutation of I91N/L211I can increase β-mannanase activity. The food-grade expression of ManBl (I91N/L211I) opens up a way to its application with biosafety.