[Background] Proteus mirabilis is an opportunistic pathogen ubiquitous in the environment and often causes infections in animals and humans. [Objective] To explore the cause of death of sick chickens in a chicken farm in Changchun city, Jilin province, and provide reference for disease prevention and control. [Methods] The pathogenic bacterial strain TSA-1 was isolated from the organs of dead young chickens and identified by Gram staining, biochemical tests, and 16S rRNA gene sequencing. Furthermore, drug sensitivity test, virulence gene detection, cytotoxicity and adhesion tests, and challenge in larvae of Galleria mellonella were performed for this strain. [Results] TSA-1 was a short rod-shaped or spherical Gram-negative bacterium, with the biochemical properties consistent with those of P.mirabilis. The 16S rRNA sequence alignment showed that TSA-1 shared the similarity of 100% with P.mirabilis. The drug susceptibility test results showed that TSA-1 was resistant to 14 antibiotics including ampicillin, tetracycline, kanamycin, and cefazolin and sensitive to 7 antibiotics including ciprofloxacin and enrofloxacin. The strain had strong biofilm formation ability and carried 8 virulence genes: ireA, ucaA, pmfA, atfA, ptA, zapA, hpmA and flhC. It showed cytotoxicity to RAW 264.7 macrophages and had strong adhesion to Caco-2 cells. In addition, it had stronger lethal effect on larvae of G.mellonella than avian pathogenic Escherichia coli O78. [Conclusion] P.mirabilis TSA-1 isolated from the organs of dead chickens has multi-drug resistance and carries a variety of virulence genes. It is the one of the major pathogenic bacteria causing chicken death and deserves more attention.