The present disclosure provides an innovative method for improving the enzyme activity of an NMN biosynthetic enzyme Nampt, and relates to the technical field of genetic engineering. A mutant protein of the present disclosure is obtained by firstly analyzing a target protein Nampt using two softwares FoldX and DeepDDG, and then predicting multiple key sites influencing the enzyme functions and finally performing the semi-rational design of the enzyme. In the examples of the present disclosure, 10 mutant strains are constructed using the designed primers according to the principle of point mutation, and 8 of the mutants have higher activity than a wild-type strain, in which the NMN yield of the mutant Nampt-V365L is increased by 62%, and the NMN yields of the mutants Nampt-S248A, Nampt-N164L, Nampt-S382M, Nampt-A245T and Nampt-A208G are increased by 34%, 27%, 27%, 22% and 17% respectively.