Abstract: Based on the continuous section temperature and salinity data obtained in the tropical East Pacific Ocean by the underwater glider developed independently by Shenyang Institute of Automation, Chinese Academy of Sciences,and on the comparisons of multiple sets of data, it is found that the temperature and salinity data observed by the underwater glider are precise and reliable, which can be widely applied for deep ocean exploitation in future. It is found for the first time that there is low salinity water mass in the middle layer of 60 to 80 m of the North Pacific Central Water (NPCW), which is located in 50-100 m layer in the areas investigated. The analysis shows that the low salinity water comes from the California Current System waters (CCS) under the water mass. The dynamic mechanism of the low salinity water mass formed in the middle layer is mainly controlled by the internal waves near the thermocline and closely related to the intensity of the internal waves. At the same time, affected by the upper barrier layer at 20 to 60 m, the low salinity water mass in the middle layer can only appear at 60 to 80 m. In this study, it is found that the internal waves and the barrier layer can affect the formation of fine structure of water masses by affecting the transmission of kinetic energy and heat energy. The relevant achievements enrich the knowledge about the response of the internal waves and the barrier layer to the upper ocean besides its important scientific value.