Endoscopic ultrasound (EUS) is unsatisfactory in distinguishing between T1a and T1b stage esophageal squamous cell carcinoma (ESCC). Consequently, the National Comprehensive Cancer Network guidelines recommend endoscopic resection (ER) as a diagnostic tool for substaging T1 stage ESCC. However, as an invasive approach, diagnostic ER is not an optimal approach especially for T1b cases as most of them might not be fully cured by ER. It is necessary to develop reliable and less invasive methods to distinguish between T1a and T1b stage ESCC. In our previous unicentral trial, we found that submucosal saline injection (SSI) significantly improved the diagnostic accuracy of EUS in differentiating between T1a and T1b stage ESCC. It can be used as an alternative to diagnostic ER for preoperative substaging T1 stage ESCC cases in remote regions where few endoscopists are able to perform diagnostic ER. The use of EUS and SSI would help T1b stage patients avoid invasive diagnostic ER. Therefore, we aim to conduct a multi-center clinical trail to examine whether SSI can improve traditional EUS accuracy in distinguishing between T1a and T1b stage ESCC.