Investigators plan to conduct the participants level, 1:1 randomized trial at the Central ICU and respiratory ICU of Xiangya Hospital in Changsha, China, from January 1, 2018 to December 31, 2019. Participants with initial diagnosis of severe pneumonia will be first treated with empiric therapy, after 48-72 hours, the participants's condition will be evaluated, and participants who will have been initially cured and died should be excluded.The participants who will have failed the initial treatment will be then randomly divided into two groups: the NGS group and the conventional treatment group. Bronchoalveolar lavage fluid will be taken by bedside bronchoscopy when the participants underwent nasal high-flow oxygen therapy, non-invasive ventilation, and invasive ventilation,or venous blood will be taken for examination. Participants in the NGS group will be tested for both NGS and conventional pathogens, however, the routine group will be only routinely tested. According to the pathogen results, participants with non-severe pneumonia will be excluded, and medication should be adjusted under the joint guidance of infectious and respiratory specialists. Then investigators will compare the first outcome: 28-day mortality, and secondary outcome: ICU hospitalization time, mechanical ventilation time, Hospitalization costs, antibiotic use costs, SOFA scores, CURB-65 scores, APACHE scores, and other clinical indicators.

Inclusion Criteria:

• Patients diagnosed with severe pneumonia between the ages of 18 and 85 years; the diagnosis of severe pneumonia is based on IDSA / ATS

Exclusion Criteria:

• < 18 years old, received antibiotic treatment within 14 days before admission, suffering from severe immunosuppression, multi-drug resistance, refractory tuberculosis, end-stage renal disease or liver disease (uremia, advanced liver cancer), pregnancy. Besides, we need to rule out patients who have been improved, died, and abandoned during the initial treatment。