Abstract: On January 21 2006, the water and mud popped out from the 978 high-pressure and water-filled cavern at the exit to Maluqing tunnel. The water amount was instantaneously 300 thousand cubic meter per hour. For ensuring the safety of construction and traffic, a 4700 m-drainage tunnel was built to release the karst water in the Maluqing tunnel. To release the karst water includes the following 4 stages: water release by boring, water release by high place hole, water release by low place drainage tunnel, automatic release of water, mud and rock due to strong rainfall. The water release mechanism is summarized based on the measured data. To release water by boring can reduce the water pressure, but the water release ability is limited because of the boring blockage. To release water by the high place hole has certain effectiveness, but, if the high place hole encounteres the cavern, contineous construction will be difficult so as to have impact on the water release ability. As the drainage tunnel connects with the cavern, when the rainfall is strong, the water, mud and rock release from the drainage tunnel. So, the cavern is ceaselessly dredged, and the function of water release is satisfied. The maximum water release is corresponding to the rainfall.