The invention which relates to the field of genetic engineering provides a constitutive expression cassette of Trichoderma reesei. The expression cassette comprises a foreign target gene, a promoter of a pyruvate decarboxylase gene (Ppdc) of Trichoderma reesei, and a terminator of the pyruvate decarboxylase gene (Tpdc) of Trichoderma reesei, wherein the nucleotide sequence of the Ppdc is represented by SEQ ID NO.1, and the nucleotide sequence of the Tpdc is represented by SEQ ID NO.2. The expression cassette which comprises the sequence of the Ppdc of Trichoderma reesei and the sequence of the Tpdc of Trichoderma reesei can express genes from animals, plant or fungi without induction, and can properly modify expression products, protein expression can be carried out with the highly efficient synthesis and secretion capability of Trichoderma reesei, the protein expression level is high, and excessive foreign proteins in the expression products can be avoided. A recombinant strain of the present invention can highly express xylanase without cellulose, and the xylanase has an important application in the papermaking industry.