ObjectiveTo comprehensively evaluate the causal relationship between gut microbiota and 9 respiratory diseases, explore whether there are common mechanisms among these 9 respiratory diseases, and delves into the potential commonalities in the nascent stage of chronic obstructive pulmonary disease (pre-COPD) by focusing on COPD, emphysema, and chronic bronchitis. MethodsMendelian randomization was used to explore the causal relationship between gut microbiota and 9 respiratory diseases, sensitivity analysis was conducted, and robustness was verified. Single nucleotide polymorphisms of common gut microbiota were mapped to genes to explore their common mechanisms. ResultsMendelian randomization studies found that 66 gut microbiota had a causal relationship with 9 respiratory diseases, and 15 gut microbiota might be potential common mechanisms for these 9 respiratory diseases, with a total of 85 potential genes. GO functional enrichment analysis found that these genes were mainly enriched in biological functions such as synaptic transmission, synaptic membrane, cytokine activity, and growth factor binding. KEGG enrichment analysis showed that these genes were mainly enriched in glutamatergic synapses, relaxin signaling pathway, calcium signaling pathway, Rap1 signaling pathway, and PI3K-Akt signaling pathway. Allisonella and Prevotellaceae might be protective factors for pre-COPD, while Howardella might be a risk factor. Ten protective genes and six risk genes for the pre-COPD were identified. ConclusionThis study further confirms the correlation between the lung-gut axis and the lung-colon connection, explores the potential common mechanisms of 9 respiratory diseases, investigates the potential mechanisms of pre-COPD based on the common gut microbiota of COPD, chronic bronchitis, and emphysema, and provides some ideas for the early prevention and treatment of COPD, and diagnosis of pre-COPD from the perspective of gut microbiota.