Objective To explore the diagnosis and clinical characteristics of atypical severe pneumonia caused by Chlamydia abortus(C. abortus). Methods Clinical data of 4 patients diagnosed with atypical severe pneumonia caused by C. abortus in a hospital from January 2021 to November 2022 were collected. Clinical characteristics, dia-gnosis and treatment, and precautions of the disease were comprehensively analyzed. Results All 4 patients were male, aged 63-73 years old, with acute onset, high fever, cough and expectoration. Three patients had a history of contact with poultry, one patient had a history of contact with abortion goat. The interval between the emerging of clinical symptoms and the onset of acute respiratory failure in 4 patients was 1-6 days, and the oxygenation index (PaO₂/FiO₂) at admission was less than 200 mmHg, which gradually decreased with the progression of the disease, active support with a ventilator was necessary. Two patients had an increase in white blood cell count, 4 had an increase in neutrophil percentage, 3 had a mild decrease in platelet count. Among 4 patients, 2, 2, 3 and 4 patients showed elevated levels of aspartate aminotransferase, alanine aminotransferase, creatine kinase, and serum creatinine respectively, 2 patients had mild hyponatremia, 4 patients showed significant increase in C-reactive protein, procalcitonin, and interleukin-6 levels. Four patients' chest CT findings showed main involvement of single or multiple lung lobes, with exudation and consolidation, and later involvement of multiple lobes of lung. The metageno-mic next-generation sequencing of bronchoalveolar lavage fluid detected the DNA sequence of C. abortus. Based on the clinical manifestations, contact history, chest CT, and metagenomic next-generation sequencing results of 4 patients, the diagnosis was C. abortus. atypical severe pneumonia. After timely adjustment of the treatment of anti-infection regimen based on doxycycline, the patients' condition improved and were discharged. Conclusion C. abortus may also cause human pneumonia, which can lead to serious clinical outcome after infection. Patient had a history of animal contact should be alert to such diseases. Metagenomic next-generation sequencing can detect C. abortus.