The invention relates to the field of microorganism genetic engineering, in particular to a strain, and a construction method and application thereof. Xanthomonas campestris is used an original strain, a key gene pigA of an xanthomonadins synthesis way is inactivated through a genetic engineering technique, a haemoglobin vgb gene is introduced, and a xanthan gum white mutant (the strain is authorized to the China General Microbiological Culture Collection Center for preservation with the preservation number being CGMCC No.14771) is constructed. White xanthan gum can be produced, synthesized xanthan gum basically excludes the xanthomonadins, side effects of unknown mutation to strains can be reduced, the usage quantity of ethanol can be massively reduced, resources and the production cost can be saved, and the strain has great industrial application value.