Background: Although PD-L1 positivity improves immunotherapy efficacy, PD-L1 testing alone is insufficient for patient selection in most malignancies. High tumor mutational burden (TMB) is an emerging positive biomarker of immunotherapy in a growing number of malignancies. In this study, a method for TMB by cancer-gene panel (CGP) was set up, by which TMB from Chinese lung adenocarcinoma samples was measured to analyze the association between TMB and genes alterations in the implications for immunotherapy. Methods: The accuracy of TMB detection by our CGP was confirmed from data of TCGA and whole exome sequencing (WES) in our clinical samples. TMB detection method by CGP was further validated from sequencing accuracy, coefficient of variation, and the limit of detection of tumor purity. The level of TMB in Chinese lung adenocarcinoma samples and its correlation with genes alterations were then analyzed. Results: In our study, TMB measured by CGP has a high correlation with that measured by WES in 31 clinical tumor samples (R²= 0.9111). Immunotherapy from the Rizvi cohort confirmed the accuracy of TMB detection by CGP. The mean level of TMB measured by CGP from 599 Chinese lung adenocarcinoma samples was similar with that from 389 TCGA lung adenocarcinoma samples (7.29 vs 7.6 mutations/Mb). We found that high level of TMB was significantly correlated with several genes alterations including TP53, KRAS and genes in DNA damage response pathway as BRCA1/2, ATR, CHEK2, POLE and MMR genes. However, low level of TMB was correlated with EGFR and ALK alteration obviously. Of note, we first discovered that the level of TMB was lower in the early stage lung adenocarcinoma compared with that in the late stage lung adenocarcinoma from Chinese clinical samples. Conclusions: TMB calculated by CGP and WES was highly correlated via analyzing data both from TCGA and our tumor samples. We found that the level of TMB was effected by several genes alterations in the analysis of our samples. We also found that the low level of TMB might imply a poor effect of mono-immunotherapy in the treatment of early stage lung adenocarcinoma. Of note, combining immunotherapy with DNA damaging agents could be a good way to improve efficacy.