Abstract: In order to study the water burst and extension law of weathered rock crack,a testing apparatus of rock min- ing-crack extension under water flow has been developed. The physical features of weathered mudstone located in dif- ferent positions were analyzed through some basic physical tests which include saturated water absorptivity,point load intensity etc. Using two groups of rock specimens according to the difference of their physical features,the composition of rock sample was tested through X-ray diffraction (XRD) experiment. The experiment of water burst and extension law of crack was conducted through setting up different water injection pressures and crack widths with the two groups of rock specimens. The study obtained the change rule of water pressure and water inflow,discussed the influence of water injection pressure,crack width and physical properties of rock on crack extension,and summarized the crack ex- tension law of weathered mudstone.