Disclosed herein are ammonia-specific 5′-XMP aminase mutants and a method for preparing the same. A mutation is introduced into the active site of glutamine-dependent catalysis in 5′-XMP aminase. The resulting 5′-XMP aminase mutant is devoid of the glutamine-dependent activity and specifically reacts with external ammonia in converting 5′-XMP into 5′-GMP. Thus, the ammonia-specific 5′-XMP aminase mutant is stabler within cells compared to the wild type, and can be useful in the industrial conversion of 5′-XMP into 5′-GMP.