Abstract: Aiming at the serious rib spalling accidents at 8107 large mining height fully mechanized caving face of Tongxin Coal Mine, combining with the field investigation, mechanical tests, numerical simulation and theoretical analysis, the mechanical model of coal wall curved slide instability is established. The research results are as follows: the main form of rib spalling is curved slide instability in soft coal seam with large mining height. Initial rupture point of rib spalling is located at the springing of top coal caving arch; the results solving curved slide safety factor by using strict Janbu procedure show that key parameter of rid control is the resistance protecting rib F_r and its range is from 1 000 kN to 2 000 kN, the control parameter of curved slipde track α and its range is from 30° to 60°. Then, the mechanical parameter of the coal and the associated parameters of caving arch are also relevant parameters; lack of working resistance and the resistance protecting rib, which caused by leak failure of hydraulic system is the main reasons of rib spalling. Detection and precise location of leak failure can be realized by YHX leak detection instrument through collecting and analyzing the high frequency sounds and vibration signal.