Abstract: Some scholars have made lots of indepth researches on the inrush of the upper loose waterbearing sand layer caused by shallow coal mining.It is well known that the sediment originates from the quaternary loose layer.In recent years,the mixed watersand outburst accidents caused by mining coal under Jurassic Thick Overburden Rock in northwest China are increasing,which has become a new and harmful geological harzard.In order to reveal the mechanism of watersand inrush in bedrock and to find some scientific preventive measures,taking the accident in the coal mining face of Huanglong Coalfield in Shaanxi Province as an example,based on the supplementary exploration results and combined with the accident process,this paper puts forward a technical discussion on the cause analysis and determination of the accident made by predecessors.It points out that this accident has the characteristics of separated layer water disaster:the function of collecting water source in separated layer space transforms the sandstone fissure water into “free” water body,which causes shortterm and highintensity water inrush,and then it also points out that the separated layer water disaster needs five conditions simultaneously,namely,rock mechanics,waterrichness,time,space and channel conditions.Based on a large number of experimental data of rock physics and mechanics,it is concluded that the overlying strata of 4-2 coal seam have the material basis of separation formation,and according to the time and space conditions,combined with the application the “Empirical Formula”,it is concluded that the separation space causing the accident should be located in the stratum range of 30.54-77.10 m above the 4-2 coal roof,and the stratum in this height range should be taken as the target stratum for waterrich evaluation.Sandstone thickness,brittleplastic ratio and structural fractal dimension are selected,and a new composite superposition method is used to draw the isogram of waterrich comprehensive index.It is proved that the surrounding rock has the waterrich (water source) condition,considering the fact that the ancient riverbed (or loose geological body) has not been found in previous exploration,and the sediment in this accident originates from waterbearing bedrock.Finally,according to the five basic conditions,some technical measures are put forward to change the formation waterrich condition or to destroy the closure condition (or time condition).Through the study of this typical case,revealing the disasterinducing mechanism of watersand burst in mining under thick overburden bedrock of Jurassic Coalfield in northwest China,it can be used for the safe mining at similar mining area in the west of China.