Background: The dynamics of the cancer genome and T-cell receptor (TCR) repertoire are largely unclear during the development of gastric cancer, especially in the early stage. In this study, we studied the characteristics of genome and tumor microenvironment changes chronologically in 15 early gastric cancer (EGC) patients and illustrated their influence on carcinogenesis. Methods: 75 gastric tissues including precancerous lesions, cancer lesions and paired reference gastric samples, and 29 peripheral blood samples were obtained chronologically from 15 patients. All the lesions were categorized into 3 groups according to its pathology type (group1: inflammation or intestinal metaplasia, group 2: low-grade dysplasia (LGD), group 3: EGC). Deep sequencing of whole-exome and CDR3 chain of TCR of each sample was performed. Tumor mutation burden (TMB), TCR repertoire diversity and clonality were evaluated based on the sequencing data. Results: Heterogeneity existed in cancer genome and TCR repertoire among different patients. During carcinogenesis, TMB increased at first and then decreased, reaching its peak at LGD, which was different from what we have known from other advanced cancer. A detailed analysis showed that only part of the driver gene mutations maintained in this process. Compared with T cells infiltrated in reference samples, tumor-infiltrating T cells shared more clones with peripheral blood T cells. Diversity and clonality of TCR repertoire didn’t show significant change with the development of cancer (Shannon index: group 1 = 6.32, group 2 = 6.06, group 3 = 6.09, p = 0.836; Clonality: group 1 = 0.19, group 2 = 0.19, group 3 = 0.17, p = 0.374). Some CDR3 clones appeared in early-stage and maintained during the carcinogenesis process. Conclusions: Our research is the first to analyze the cancer genome and TCR repertoire changes from both the spatial and temporal perspectives during the development of early gastric cancer. Somatic mutations appeared in the very early stage of gastric cancer and may induce the immune microenvironment changes in gastric mucosa.