The diameter and depth of the launch shaft of Qianwan subsea shielddriven tunnel in Shenzhen are 18m and 28.54m respectively. The lauch shaft is sunk by means of open caisson method. During the sinking of the launch shaft, underwater bottomsealing is conducted. With a thickness of 4.5m, the concrete casting of the bottomsealing construction belongs to hugevolume concrete casting construction. It is worthy to make study on the concrete casting construction so as to fulfill the bottomsealing with high quality and as per the schedule. On basis of the actual conditions of the project and the relevant theoreatical calculations, the article presents the construction technology of the hugevolume and largecrosssection underwater concrete casting for the bottomsealing of the launch shaft situated in coastal ground.