Abstract: In order to identify the cause of pig morbidity and mortality in a large-scale pig farm in Sichuan region and to carry out effective prevention and control, in the test, four samples of diseased and dead pig lung tissues were collected for isolation of bacteria; and the isolates were subjected to colony morphology observation, Gram staining and microscopic examination, PCR identification of Streptococcus suis, serotype identification, detection of major virulence genes, drug sensitivity test and drug resistance gene detection. The results showed that a total of 4 strains of bacteria were isolated from the diseased material; all 4 isolates showed grayish-white, acicular, rounded colonies in TSA solid medium; microscopic examination after Gram staining showed blue-purple, round or oval, chain arrangement Gram-positive bacteria, which were conformed to the morphologic characteristics of Streptococcus suis, and were named as SYS9, SYS14, SYS15, and SYS16, respectively. PCR identification and serotype identification of Streptococcus suis determined that all four isolates were Streptococcus suis type 3. The four strains of Streptococcus suis were classified into two virulence genotypes; the virulence genotypes of strain SYS9 were mrp+epf+sly+fbps+cps-orf2+, and those of SYS14, SYS15, and SYS16 were mrp+epf+sly+fbps+cps-orf2+, and those of SYS15 and SYS16 were mrp+epf+sly+fbps+cps+orf2+. All were resistant to tetracycline and tylosin; all were susceptible to amoxicillin（penicillins）, florfenicol, chloramphenicol（aminoglycosides）, linezolid（oxazolidinones）, cefpirome, cefadroxil, and cephradine（cephalosporins）, and ciprofloxacin and levofloxacin（quinolones）. Except for strain SYS9, which was resistant only to tetracycline and tylosin, the remaining three strains were 3-or more-drug resistant, with strain SYS15 being 6-drug resistant. None of the four strains of Streptococcus suis detected ermA, mefA, msrD, fexA, fexB, aph（3′）-Ⅲa, ant（6′）-Ⅰa, and aph（2′）-Ⅰb genes; the SYS9 strain detected the tetO gene and the ermB gene; the SYS14 strain detected tetM, tetO, ermB, aac（6′）-aph（2″） and optrA genes, and the SYS15 strain detected tetM, tetO, tetL, ermB, aac（6′）-aph（2″） and optrA genes, and SYS16 strain detected tetM, tetO, tetL, ermB, aac（6′）-aph（2″）, optrA and Lsa（E） genes. The results indicated that the cause of pig morbidity and mortality in this large-scale pig farm was Streptococcus suis type 3 infection, which could be treated with penicillins, amidol, oxazolidinones, cephalosporins, and quinolones.