Chronic obstructive pulmonary disease (COPD), which predominantly affects middle-aged and elderly individuals, is associated with a significantly reduced quality of life and often triggers various other pulmonary conditions. Lung cancer, as one of the most prevalent and deadly pulmonary malignancies worldwide, poses a severe threat to global public health. The risk of developing lung cancer is markedly higher in COPD patients compared to the general population, indicating numerous associations between the two conditions that warrant in-depth investigation. Although a substantial body of research has explored the relationship between COPD and lung cancer, studies focusing on the molecular mechanisms underlying their connection remain limited. This article reviews the latest research progress on the mechanisms of COPD complicated by lung cancer from four perspectives: the role of chronic pulmonary inflammation, programmed cell death, genetic and molecular interactions, and dysbiosis of the pulmonary microbiome. The aim of this article is to provide new insights and references for the prevention and therapeutic strategies of COPD complicated with lung cancer.