The invention relates to a method for designing gene sequences, in particular to a human aldehyde dehydrogenase 2 sequence designed according to a preferred codon of pichia pastoris. The sequence is characterized in that a DNA molecule is designed according to the preference of the pichia pastoris, and a code of the molecule has the same nucleotide sequence as a protein coded by a human aldehyde dehydrogenase 2 gene ID 217 on an amino acid level. The human aldehyde dehydrogenase 2 gene sequence designed according to the preferred codon of pichia pastoris is realized by the following steps: on the basis of a human ALDH2 gene (gene ID 217), UTR and signal peptide are deleted; enzyme cutting sites of an incision enzyme EcoR I and an incision enzyme Not I are added; a codon GCG for coding Ala is replaced by GCU according to the use method of the pichia pastoris codon, and codons CGC and CGG for coding Arg are replaced by AGA, AGG, CGU and CGA; and altogether 28 bases are modified to obtain a target sequence ALDH2. The gene sequence can express with high efficiency in the pichia pastoris, can be applied to preventing alcoholism and protecting livers and has the effects of preventing alcoholism and protecting livers.