The heterogeneity of lung adenocarcinoma (LUAD) indicated that target therapies and immunotherapies may not be effective in all patients. The exploration of the feature of the immune landscape of different gene mutations may provide novel perspectives. In this study, we obtained LUAD samples from The Cancer Genome Atlas. By applying ESTIMATE and ssGSEA, KRAS-mutated group was discovered to be associated with lower immune infiltration, lower expression of immune checkpoints, especially, a lower abundance of B cell, CD8+ T cell, dendritic cell, natural killer cell, and macrophage, higher abundance of neutrophil and endothelial cell. Through ssGSEA, we found that the process of antigen-presenting cell co-inhibition and co-stimulation were inhibited, cytolytic activity and human leukocyte antigen molecules were downregulated in the KRAS-mutated group. And KRAS mutation is negatively related to antigen presentation and procession, cytotoxic lymphocyte activity, cytolytic activities, and cytokine interaction signaling pathway via gene function enrichment analysis. Finally, 24 immune-related genes were identified to establish an immune-related gene signature with excellent prognostic prediction capacity, whose 1-, 3- and 5-year AUCs were 0.893, 0.986, and 0.999. Our findings elucidate the features of the immune landscape of KRAS-mutated groups and successfully established a prognostic signature on the basis of immune-related genes in LUAD.