The invention provides a protein encoded by idiotoxin Der f4 gene of dermatophagoides farinae and an application of the protein. A prokaryotic expression plasmid pET-28b-Der f4 and a eukaryotic expression plasmid pGAPZalpha-A-Der f4 can be respectively constructed according to the gene, and then the recombinant protein of the gene is prepared. The protein and the application thereof have the following advantages that a full-length gene sequence of the idiotoxin Der f4 gene of dermatophagoides farinae is obtained, the recombinant protein is prepared according to the gene sequence, and the encoded recombined protein is high in purity, good in specificity and rich in yield. The protein provided by the invention can be applied to diagnosing whether a patient suffers allergic diseases caused by the fourth component of the idiotoxin of dermatophagoides farinae.