An extralarge karst cave is encountered during the construction of Baixugong tunnel. The geological conditions of the karst cave are investigated and the strike and joint distribution of the rock mass are surveyed. Countermeasures are determined according to the actual conditions of the karst cave and the geological data collected. Technologies, including executing pile foundations at the bottom of the karst cave, setting pile caps and supporting walls on foundations, using transverse connection beams to combine the karst cave bottom and the tunnel, and clearing and “shotcreting and rock bolting” reinforcement of the side wall and roof of the karst cave, are proposed. In the end, the tunneling crosses through the extralarge karst cave successfully.