The invention relates to a long-chain dibasic acid with low content of monobasic acid impurity and a production method thereof, in particular to the preparation of a long-chain dibasic acid producing strain by means of directed evolution and homologous recombination, and to the production of a long-chain dibasic acid with low content of monobasic acid impurity by fermentation of said strain. The invention relates to a mutated CYP52A12 gene, homologous gene or variant thereof, which, relative to GenBank Accession Number AY230498 and taking the first base upstream of the start codon ATG as −1, comprises a mutation. The invention relates to a strain comprising said mutated CYP52A12 gene, homologous gene or variant thereof wherein when the strain is fermented to produce a long-chain dibasic acid, the content of monobasic acid impurity in the fermentation product is significantly reduced.