Abstract: Objective To explore the diagnostic criteria and physiopathological features of severe bacterial pneumonia in rats. Methods A total of 60 Sprague Dawleg rats were randomly divided into 3 groups, namely the model group (n=24), observation group (n=24), and control group (n=12). The rats in the former two groups were intratracheally instilled with Klebsiella pneumoniae suspension at different doses, while those in the control group received intratracheal administration of 1 ml saline. On the 2nd, 4th, and 6th days after intratracheal instillation, 1/3 of the rats in each group were killed to determine the hemodynamics, arterial blood gas (ABG), peripheral hemogram, leukocytes and neutrophils in the bronchoalveolar lavage fluid (BALF), followed by pathological examination of the lung. Results Obvious hemodynamic changes occurred in the rats of the model group on the 5th day and ABG changes appeared on the 4th day. The changes in peripheral hemogram, leukocytes and neutrophils in the BALF, and lung pathology of the rats in the model group were more obvious than those in the observation group on each time point. As the time prolonged, the changes in the hemodynamics, ABG, peripheral hemogram, leukocytes and neutrophils in the BALF and pathology of the lung exacerbated in the model group and observation group, but all these indices in the control group remain unchanged after bacterial inoculation. Conclusion The severity of pneumonia in rats increases with the dose increment of the administered Klebsiella pneumoniae suspension, which mimics the pathological changes in severe bacterial pneumonia in human.