Early diagnosis of leptospirosis of pulmonary diffuse hemorrhage type (PDH) is of crucial importance in saving patients. To develop a sensitive and specific method for diagnosis, a genomic library of the main pathogen of PDH, L. interrogans serovar lai strain 017, was constructed with the plasmid vector pUC9. Recombinant plasmids which have homologous fragments of pathogenic leptospires were screened from the bank. A recombinant plasmid, designated pCX7, could detect 1.7 kb fragment of strain 017, 9.0 kb of strain 601 and 30.0 kb of strain Hebdomadis, respectively, without cross hybridization with nonpathogenic leptospires such as L. biflexa strain Patoc I and Leptonema illini. The recombinant plasmid pCX7 could detect pathogenic leptospires which are the main pathogens endemic to Sichuan Province.