Lung adenocarcinoma (LUAD) has become the most common pathological type of lung cancer in recent years. However, the molecular mechanism of LUAD remains unclear. To reveal the laws in the occurrence and development of LUAD, this project first collects 676 differentially expressed genes related to LUAD and the corresponding proteins encoded by the genes, resulting in a complex protein-protein interaction (PPI) network. The traditional analysis methods focus on the gene interaction relationships or the important genes separately, ignoring the fact that some important target genes may take effect jointly. In contrast, this project adopts a new analysis method named the overlapping gene analysis method to screen out the closely interacted and important genes. There into, the hub genes are first discovered according to the node importance index, and then the PPI network is divided into multiple communities. The overlapping genes are some genes belonging to both the hub genes and the genes in the same community, regarded as gene groups. Survival analyses to the gene group show that only PLK1, CCNB1, and CDK1 participate in the prognosis of LUAD, and relate to the pathological stage of LUAD. These demonstrate that the three genes are extremely important in the cell cycle, which is confirmed by the following enrichment analyses. Besides, some significant correlations are observed among the three genes, which provide a clue that the three genes may cooperate in the occurrence and development of LUAD. This finding provides possible clinical targets for the diagnosis and treatment of LUAD.