Abstract: There is high pressure and abundant water in MaoBa syncline where Yuanliangshan tunnel lies,that brings a great many difficulties for design and construction of the tunnel. This paper introduced the observation of water pressure. Through theoretic analysis of the observed data, the numerical value of water pressure in MaoBa syncline and the reduction of water pressure after grouting are calculated.The predicted shape of dissolved cavity is used to supervise the next grouting.