Abstract: In order to find out the cause of diarrhea death in a donkey farm in Liaocheng region, Shandong Province, the pathogens of Equine rotavirus and Equine coronavirus were detected by RT-PCR in diseased samples of liver, spleen, kidney and other tissues of dead donkeys. The bacteria in the material was isolated and purified, and the isolated bacteria were identified by Gram staining, morphological characteristics observation, biochemical identification and 16 S rDNA gene sequence analysis. Drug sensitivity test, virulence gene detection and mouse pathogenicity test were performed. The results showed that both Equine rotavirus and Equine coronavirus were negative in the infected material. A strain of bacteria was isolated from liver tissue, which could grow on SS agar culture medium, LB agar culture medium, blood AGAR medium and MAC agar culture medium, and was confirmed to be short rod or ball rod shaped Gram-negative bacilli by microscopic examination, and its biochemical characteristics were consistent with those of Proteus mirabilis. The 16 S rDNA gene sequence of the strain shared 99.87% to 100% nucleotide similarity with 10 strains of Proteus mirabilis in GenBank, which further confirmed that the strain was Proteus mirabilis and named LC1（GenBank accession number: ON018729）. LC1 strain was sensitive to ampicillin, norfloxacin, ofloxacin, cefazolin, cefoperazone, cefotaxime, aztreonam, levofloxacin, amikacin, spectinomycin, gentamicin and ciprofloxacin. It was resistant to erythromycin, clindamycin, imipenem, doxycycline, and polymyxin B. It was mediated to cephalexin. A total of 9 virulence genes were detected in LC1 strain, including MrpA, FliL, RsmA, HpmA, AtfA, UreC, ZapA, PmfA and RsbA. The mice infected with LC1 strain developed disease 6 hours later, began to die 8 hours later, and all died within 24 hours. Proteus mirabilrs was isolated from the liver and lung of dead mice again. This indicated that a strain of Proteus mirabilis was successfully isolated from the dead donkey foal, which was an important pathogen causing the morbidity and death of the donkey foal.