Abstract: With the increase of mining depth of coal seam year by year,more and more subcritical working faces are coming into being. The height of water flowing fractured zone is a key parameter to realize water-preserved mining,but the heights of water flowing fractured zone in subcritical working faces are lower than those in critical working faces. In order to further study its causes,the sensitivity of influencing factors of height of water flowing fractured zone and the relationship between the height of water flowing fractured zone and size of working face were studied by means of theoretical analysis,similar simulation and numerical simulation. The definition and distinguishing method of critical min-ing degree of overburden failure were proposed. The results show that the influence of size of working face on the height of water flowing fractured zone is next only to mining thickness. When the size of working face is small enough, the overburden failure does not develop. When the size of working face increases to a certain value,the overburden failure only forms caved zone. When the size of working face continues to increase,the overburden failure forms frac-tured zone and the height of water flowing fractured zone increases with the increase of size of working face. When the height of water flowing fractured zone develops to the maximum value,the height of water flowing fractured zone will not continue to increase with the increase of size of working face. The stage in which only caved zone formed in over-burden failure process was defined as super-subcritical mining of overburden failure. The stage in which the fractured zone formed and the height of water flowing fractured zone increases with the increase of size of working face was de-fined as subcritical mining of overburden failure. The stage in which the height of water flowing fractured zone reaches the maximum value and no longer increases with the increase of size of working face was defined as critical mining of overburden failure. The size of working face was defined as the critical size of working face when the height of water flowing fractured zone reaches the maximum value for the first time. When the size of working face is smaller than the critical size of working face,the mining degree of overburden failure is subcritical. When the size of working face is lar-ger than the critical size of working face,the mining degree of overburden failure is critical. The main influencing fac-tors on the critical mining degree of overburden failure are the size of working face,mining thickness,mining depth,o-verburden mechanical properties,overburden structural characteristics and overburden breaking angle.