A mutated xylanase gene with a broad pH range of reaction includes a forty-first amino acid or a twenty-first amino acid generated from transforming asparagine to aspartic acid so as to form the mutated xylanase gene. A site-specific mutagenesis method includes the step of: mutating the forty-first amino acid or the twenty-first amino acid of the xylanase gene by transforming asparagine to aspartic acid so as to form the mutated xylanase gene.