The present invention relates to a long-chain dibasic acid with low content of long-chain dibasic acid impurity of shorter carbon chain, to the preparation of a long-chain dibasic acid producing strain by directed evolution of POX gene and homologous recombination, and to the production of a long-chain dibasic acid with low content of long-chain dibasic acid impurity of shorter carbon chain by using the strain. The present invention also relates to a strain containing a mutated promoter, wherein, when a long-chain dibasic acid is produced by fermentation of this strain, the content of the acid impurity of shorter carbon chain in the fermentation product is significantly reduced.