The invention discloses a strain for optimizing a fermenting composition of erythromycin, a construction method and application thereof. The strain is saccharopolyspora erythraea which is classified and named Saccharopolyspora erythraea; and the preservation number of the strain is CGMCC2286. Through a means of metabolic engineering, erythromycin is strengthened and is subjected to biosynthesis to modify enzyme; in particular, the strength of C12-hydroxylase of a macrocyclic framework and mycarose methylase; an reconstructed erythromycin promoter is introduced; and through different mixture ratio of gene dosage, the strength and proportion of the hydroxylase and the methylase are adjusted, so as to obtain the strain for only producing a main effective composition of the erythromycin, namely erythromycin A. The strain is adopted to ferment, can reduce impurity compositions, even fully remove the impurity compositions of erythromycin A, erythromycin B and the like, simplify a production process and reduce production cost.