Xiang'an sub-sea tunnel is the first large section sub-sea tunnel in China, and will cross the weak stratum with abundant water. As the low stabilizing capacity of the surrounding rocks, water inrush is the maximal risk during the sub-sea tunnel construction. In this paper, based on analyzing the excavation method of the tunnel and results of on site monitoring measurement, we studied the excavation method,drainage technique and pre-reinforcement method of the tunnel when crossing the weak stratum with abundant water, and offer technological support for construction.