Background: Tumor mutation burden (TMB) has been established as a biomarker for response to immune therapy and prognosis in various cancers. However, the correlation between TMB and immune microenvironment remains unwell studied, especially in urothelial carcinoma. This study was aimed to investigate the relationship between TMB and other immunotherapy related biomarkers, including genetic alterations, APOBEC signature, microsatellite instability (MSI), PD-L1 expression and immune cell infiltration in urothelial carcinoma. Methods: 131 patients with urothelial carcinoma admitted from October 2018 to May 2020 were included. Total DNA was isolated from FFPE or fresh tissues. Mutation profiles, APOBEC signature and MSI scores were obtained by next-generation sequencing based a 642 cancer genes panel assay. PD-L1 expression, CD8+ T-cells and tumor-infiltrating lymphocytes density were evaluated by immunohistochemistry. The correlation was analyzed by Wilcoxon signed-rank test. Results: The mutation landscape showed that TP53 mutation is the most common alterations (n = 64/131, 48.9%), followed by KMT2D alterations (n = 49/131, 37.4%), KDM6A mutations (n = 42/131, 32.1%), MUC17 mutations (n = 42/131, 32.1%). The median TMB was 5.06 Muts/Mb (0-118 Muts/Mb). 2 of 131 patients showed MSI-H, who exhibited a much higher TMB (41, 118 Muts/Mb). Further analysis showed that TMB in the patients with certain gene mutations (such as TP53, KMT2D, KDM6A and MUC17) was significantly higher than those wild type ones (p < 0.05). Meanwhile, the high APOBEC-enrichment group has a higher TMB than the low APOBEC-enrichment group (p = 0.045). Furthermore,we observed that the patients with a higher PD-L1 expression (n = 28/131, 21.4%, at a combined positive score cut-off value of 10) also showed a significantly higher TMB (p = 0.016), and TMB in the patients with higher density of CD8+ T-cells (n = 42/131, 32.1%, at a cut-off value of 5%) was also significantly higher than that of the group with lower density of CD8+ T-cells (p = 0.039). Conclusions: This study provides new insights into the correlation between the TMB and the immune microenvironment in urothelial carcinoma. The result may be a reference to immunotherapy.