With 19 groups of data associated with inrush or bursting water from karst cavity, obtained from Yichang-Wanxian Railway Maluqing Tunnel field test, conceptual models of Karst Water Burst Flood (KWBF) was brought forward by statistical analysis and phenomenological induction. KWBF is super-large amount of artesian water together with mud, which statically deposited in karst cavity, suddenly released into tunnel. KWBF has 3 kinds of characteristic: large scale of cavity water, high hydraulic pressure, high bed load content, dynamic supply of ground water, strong destroyed power; no foreboding, paroxysmal and intermittence, antinomy between paroxysmal and arrearage; force majeure, difficulty forecast, etc. Forecasting of Karst Water Burst Flood is conducted with rainfall seeping theory and statistical analysis method, while the base data are obtained from field quantity observation technical such as rainfall, hydrograph measure. Because of pressure dispersion between inside and outside of the cavity, seepage enlarge is evolved into conflux so that deprive groundwater from aquifer aqueduct and fracture, so, the water together with mud release suddenly from karst into tunnel. The key effect factor on KWBF is water count and silted or filling degree of conflux aqueduct. It is the essential numerical criterion that the daily rainfall must be less than 20mm if construction in the tunnel after the karst cavity has been broken through. To evade risk during construction temporary stage and permanent working stage, measure of energy releasing and pressure reducing (ERPR) must be accept. In addition, it was suggested that the key parameter of risk control for karst tunnel is water count and its cumulating velocity, mechanics characteristic of bed load filling in the conflux aqueduct.